All terms

651 storage glossary entries in alphabetical order, each with a short definition. Open a term for relations, advantages, limitations, numbers, and sources.

Machine-readable: /data/index.json and llms-full.txt. Derived tables: /datasets/.

0–9

3D NAND: Flash memory that stacks cells vertically in layers to increase density without shrinking planar features alone.
3D XPoint: A cross-point non-volatile memory technology announced by Intel and Micron, distinct from NAND flash in cell physics.
4Kn native sectors: 4Kn exposes 4096-byte logical sectors and 4096-byte physical sectors without 512-byte sector emulation.
512e emulation: 512e exposes 512-byte logical sectors over 4096-byte physical sectors.

A

Acting set (Ceph): In Ceph, the acting set is the ordered OSD list responsible for a placement group at a particular map epoch.
Active LBA range: The specified logical-block address interval available to a storage test or its preparation workload.
Active-zone limit: An active-zone limit caps the zones simultaneously in implicitly open, explicitly open or closed states in a zoned namespace.
Adaptive Read-Retry: Adaptive Read-Retry selects shorter NAND precharge timing from profiled conditions while reserving error-correction margin.
Adaptive Replacement Cache: Adaptive Replacement Cache adjusts a recency/frequency balance using resident lists and nonresident histories.
Adaptive scrubbing: Adaptive scrubbing adjusts the rate or schedule of proactive storage verification in response to observed conditions or estimated error risk.
Additional sense code: The additional sense code (ASC) is an eight-bit SCSI sense-data field that refines the sense key and is interpreted together with its qualifier.
Additional sense code qualifier: The additional sense code qualifier (ASCQ) is an eight-bit SCSI sense-data field interpreted with the ASC to identify a more specific condition.
Advanced Format (4K): Advanced Format is a disk-sector architecture expanded beyond legacy 512-byte sectors, with 512e and 4Kn as its familiar 4K variants.
Advisory file lock: An advisory file lock coordinates access among cooperating participants without itself preventing access that bypasses the locking protocol.
AHCI: Advanced Host Controller Interface, the PCI class driver model SATA SSDs and HDDs used before NVMe dominance.
Air-gapped backup: Offline copies physically isolated from production networks to resist ransomware.
All-flash array: A storage array built entirely from flash modules without HDD tiers.
ALUA: Asymmetric Logical Unit Access is the SCSI mechanism for reporting logical-unit access states through target port groups.
ANA group: An ANA group is a set of NVMe namespaces whose ANA access states change together for a given controller.
Annual replacement rate: Annual replacement rate (ARR) counts recorded device replacements relative to device exposure, scaled to one year.
Annualized failure rate (AFR): Annualized failure rate (AFR) expresses observed failures relative to drive-time exposure, scaled to a year, and is not inherently a one-year failure probability.
Anti-entropy repair: Anti-entropy repair compares replica state over a selected scope and synchronizes differences to reduce divergence.
Application quiescing: Application quiescing is application-specific preparation and temporary restriction of writes to establish a defined state for snapshot or backup capture.
Application-consistent snapshot: An application-consistent snapshot captures an application's data through a coordinated procedure that satisfies that application's consistency requirements.
Areal density: Bits stored per unit of disk surface area, the central metric driving HDD capacity growth.
Asymmetric Namespace Access: Asymmetric Namespace Access is the NVMe mechanism for reporting namespace access states through different controllers.
Asynchronous commit: Asynchronous commit is an acknowledgment mode that can report a transaction as successful before its recovery log has reached durable storage.
Asynchronous DRAM Refresh: Asynchronous DRAM Refresh is a platform power-fail mechanism that drains covered memory-controller write-pending buffers to persistent memory without protecting dirty CPU caches.
Asynchronous replication: Asynchronous replication allows primary-side completion without waiting for the corresponding update to reach a specified processing stage on a replica.
ATA SMART: ATA SMART is the ATA interface for retrieving drive self-monitoring attributes, status and logs from supporting devices.
Atomic rename: Atomic rename is a filesystem namespace operation whose successful replacement of an existing destination changes the file reached through that pathname without exposing an intermediate missing destination to concurrent…
Authenticated encryption: Authenticated encryption combines confidentiality with cryptographic integrity protection for the protected message.
Average precision: Non-interpolated average precision summarizes ranked prediction scores by weighting precision at each threshold by the corresponding increase in recall.

B

Background fsck: Background fsck is a filesystem-specific consistency-checking mode that performs permitted repairs while the filesystem remains in use.
Backup: A backup is a retained copy of data or software intended to support later recovery.
Backup manifest: A backup manifest is a machine-readable inventory of expected backup contents and associated verification metadata.
Backup verification: Backup verification checks stored backup material against specified integrity and completeness rules.
Bad block management: Firmware maps out factory and grown NAND defects so hosts see a contiguous logical address space.
Barium ferrite tape: Modern tape coatings enabling higher coercivity and capacity in LTO generations.
BD-R HTL: BD-R HTL is a recordable Blu-ray type whose written marks have lower optical reflectivity than the unrecorded material.
BD-R LTH: BD-R LTH is an organic-dye recordable Blu-ray type whose written marks have higher optical reflectivity than the unrecorded material.
Bit error rate: Bit error rate (BER) expresses erroneous bits relative to the total bits considered at a specified observation point.
Bit-patterned media: A proposed HDD technology isolating magnetic islands lithographically instead of continuous grains.
Block storage: Fixed-size addressable blocks presented to hosts or hypervisors without file semantics.
Bloom filter: A compact approximate-membership structure that can reject absent keys but may report a false positive, without false negatives for correctly inserted keys in an intact standard filter.
Blu-ray data archive: A Blu-ray data archive stores retained digital content on recordable BD-R or rewritable BD-RE optical media.
Bounded crash testing: Bounded crash testing systematically explores recovery behavior within an explicitly limited set of workloads, initial states and crash points.
Btrfs: Btrfs is a Linux filesystem built around copy-on-write B-trees, with integrated snapshots, checksumming and multi-device storage.
Btrfs NODATASUM: Btrfs NODATASUM marks file data as excluded from the filesystem's data checksumming.
Btrfs read-only subvolume: A Btrfs read-only subvolume is a subvolume whose ro property prevents modification of its file contents and directory hierarchy.
Btrfs tree log: The Btrfs tree log is a per-subvolume recovery structure that can persist file or directory changes for fsync without a full filesystem transaction commit.
Bubble memory: Non-volatile magnetic memory using cylindrical domains; briefly shipped before cheap DRAM and flash.
Buffered I/O: Buffered I/O is file access through an operating-system cache, allowing transfers between an application and storage to be decoupled.
Byte-addressable storage: Byte-addressable storage exposes individual byte locations rather than only block-sized addresses, without by itself specifying atomicity or durability.

C

Cache pollution: Cache pollution occurs when low-reuse entries displace entries that would otherwise serve useful cache hits.
CD block error rate (BLER): CD BLER counts blocks containing detectable errors per second at the input to the C1 decoder.
CFexpress: A removable card standard using PCIe and NVMe for high-end cameras and embedded recorders.
Chain replication: Chain replication orders replicas so that updates enter at a head, propagate along the chain and complete at a tail that also serves queries.
Charge trap flash: A flash cell design that stores charge in localized traps within an insulating material rather than on a conductive floating gate.
Checkpoint starvation: Checkpoint starvation is sustained inability to finish or reset a database log checkpoint because the required progress conditions keep being obstructed.
Checksum: A checksum is a value computed from specified data and later compared with a reference value to detect changes.
Chunk fragmentation: Chunk fragmentation is the dispersion of chunks needed by a deduplicated restore across storage containers or an order poorly matched to the restore sequence.
CKD DASD: Count-key-data block layout used on classic mainframe disks unlike fixed SCSI sectors.
Class prevalence: Class prevalence is the proportion of observations belonging to a specified class in a defined population or evaluation sample.
Clear sanitization: Clear sanitization uses logical operations across all user-addressable storage locations to resist simple, non-invasive recovery through the device interface available to the user, such as the host interface.
Close-to-open consistency: Close-to-open consistency is a file-sharing cache policy that combines writeback at close with validation at a later open to support visibility of updates between sequential users.
Closed-loop load: A closed-loop load model uses a fixed client population whose next request follows completion of a previous request, optionally after think time.
Codeword (storage ECC): A storage ECC codeword is an encoded unit representing a data chunk together with error-correction redundancy.
Cold archive tier: Offline or very low-cost storage tier optimized for rare reads and long retention.
Compare-and-swap: Compare-and-swap is an atomic conditional update that replaces a stored value only if it matches an expected value and reports the comparison outcome or observed value.
Compensation log record: An ARIES compensation log record describes a rollback action for redo and records where undo processing should continue.
Completion latency (fio): Completion latency in fio is the engine-dependent interval from submission of an I/O unit to observation of its completion.
Computational storage drive: An SSD or NVMe device that runs limited compute near data to reduce data movement.
Concurrent positioning range (ATA): An ATA concurrent positioning range is a contiguous logical-block range reported as supporting positioning concurrently with other reported ranges.
Conditional write: A conditional write changes a resource only when a server-evaluated precondition, such as a matching entity tag or the absence of a current object, holds.
Consumer SSD: An SSD intended for personal computers and other client devices rather than shared server workloads.
Container capping: Container capping limits the old containers referenced by a backup segment by storing selected duplicate chunks again.
Content-defined chunking: Content-defined chunking divides a byte stream using content-dependent boundary rules rather than only fixed offsets.
Conventional magnetic recording (CMR): Perpendicular magnetic recording with non-overlapping tracks so any sector can be rewritten in place.
Coordinated omission: Coordinated omission is a measurement bias in which a workload generator or sampler misses observations because its activity depends on the slow responses being measured.
copy_file_range: copy_file_range is a Linux system call that copies a requested file range in the kernel and reports the number of bytes actually copied.
Copy-on-abundant-write: Copy-on-abundant-write is a research cloning policy that buffers changes to postpone unsharing until enough work accumulates to justify copying.
Copy-on-write (storage): A technique that writes modified blocks to new locations instead of overwriting in place.
Crash consistency: Crash consistency is the property that persistent state remains within an application's allowed recovery states after an unexpected interruption.
Crash-test oracle: A crash-test oracle is a checker that decides whether a recovered state satisfies the specified correctness and durability contract.
CRUSH: CRUSH is a deterministic, pseudorandom placement algorithm that maps an identifier to storage targets using a weighted cluster hierarchy and placement rules.
Cryptographic erase: Cryptographic erase sanitizes the keys protecting encrypted target data so that recovering the plaintext becomes infeasible.
Cryptographic hash function: A cryptographic hash function deterministically produces a digest and is designed to resist preimage, second-preimage and collision attacks.
Cryptographic nonce: A cryptographic nonce is an input value whose required uniqueness scope is specified by the cryptographic construction that uses it.
CXL memory expansion: Compute Express Link attaches byte-addressable memory or accelerators on PCIe, blurring DRAM and storage tiers.

D

DAT cartridge: Digital Audio Tape format adapted for small-office digital backup in the 1990s.
Data deduplication: Data deduplication replaces duplicate content with references to shared stored blocks or chunks within a defined matching scope.
Data sanitization: Media sanitization is a process that makes recovery of target data infeasible for a specified level of effort.
Data scrubbing: Data scrubbing is proactive verification of stored data to discover errors before ordinary access requires the affected content.
Database checkpoint: A database checkpoint records engine-defined recovery state to bound or reduce the work needed after a crash.
Database write amplification: Database byte write amplification is storage-engine output bytes written divided by logical input bytes over a stated interval and accounting boundary.
Deallocated read behavior: The interface-defined data or error returned when a host reads a deallocated logical block.
Degaussing: Degaussing uses a magnetic field to disrupt recorded magnetization and can sanitize suitable magnetic media when the process is matched to the medium.
Degraded array: A degraded array is a redundant storage array operating with less than its configured healthy redundancy because one or more members are unavailable or not fully synchronized.
Delayed allocation: A filesystem allocation strategy that defers choosing physical storage locations for data already accepted into its buffered write path.
Delete marker: A delete marker is a payload-free S3 version that makes a key appear deleted when the marker is current.
Delete persistence latency: In delete-aware LSM research, delete persistence latency is the delay from inserting a tombstone to removing the entries it invalidates through compaction.
Destroy sanitization: Destroy sanitization makes target-data recovery infeasible even with current state-of-the-art laboratory techniques and leaves the medium incapable of storing data again.
Device-managed SMR (DM-SMR): An SMR HDD that presents a conventional LBA interface while the firmware relocates writes across shingled bands.
Differential backup: A differential backup captures changes accumulated since its designated full-backup base, rather than only changes since the preceding differential backup.
Differential observability: Differential observability is a difference between entities' observations of the same system's health.
Direct Access: Direct Access is a Linux mechanism that lets suitable memory-like storage serve file data without an intervening page-cache copy.
Direct I/O: Direct I/O is a file-access mode, such as Linux O_DIRECT, that requests I/O through user-space buffers while minimizing use of the local operating-system page cache.
Direct-I/O alignment: Direct-I/O alignment is the per-file constraint on memory addresses, file offsets and I/O segment lengths used for direct transfers.
Directory fsync: Directory fsync is synchronization of a directory descriptor to persist directory state, including changes to the names through which files are reached.
Discard: A block-storage request indicating that specified logical ranges are no longer needed by the caller.
Disk defect skipping: Disk defect skipping formats a record around a known defective medium region so that normal reads bypass a gap instead of storing customer data there.
Disk shelf: An enclosure providing power, cooling, and SAS/NVMe connectivity to many drives.
Distributed snapshot: A distributed snapshot records a coordinated global state of participating processes and their communication channels.
Distributed spare: A distributed spare is replacement capacity spread across multiple devices rather than reserved on a single dedicated spare device.
DLT: Digital Linear Tape, a mid-range backup cartridge family superseded by LTO in many sites.
dm-crypt: dm-crypt is a Linux device-mapper target that transparently encrypts and decrypts block-device I/O using the kernel cryptographic API.
dm-integrity: dm-integrity is a Linux device-mapper target that stores per-block integrity tags and can generate and verify them when configured with an internal hash.
dm-integrity recovery mode: dm-integrity recovery mode is a Linux device-mapper access mode that bypasses integrity checks and journal replay while disallowing writes through the mapping.
dm-verity: dm-verity is a Linux device-mapper target that verifies a read-only block mapping against a trusted cryptographic root hash.
Drive firmware revision: A drive firmware revision is the vendor-assigned version identifier reported for the firmware running on a storage drive.
Drive-day: A drive-day is one drive observed in operation for one day, used as a unit of fleet exposure.
Drum memory: Early rotating magnetic drum storage where data was recorded on the outer surface of a metal cylinder.
Dual-actuator HDD: A dual-actuator HDD is a hard drive with two independently positioning head assemblies that can service different data regions concurrently.
Dual-port drive: A drive with two independent data paths to the same media for multipath HA in arrays.
Dual-stage actuator (HDD): A dual-stage HDD actuator combines a coarse positioning stage with a fine positioning stage to control the read/write head.
DVD PI Sum8: DVD PI Sum8 counts parity-inner rows containing errors over eight consecutive ECC blocks before correction.
DWPD: Drive Writes Per Day rating expresses how many full-capacity writes an enterprise SSD can sustain daily for a warranty period.
Dynamic layout score: Dynamic layout score is a workload-specific locality measure based on the fraction of traced filesystem block requests that are contiguous in logical block address space.

E

E1.S: A short EDSFF SSD form factor aimed at 1U servers with power and thermal profiles between M.2 and U.2.
Early retention loss: Early retention loss is the rapid initial charge-loss behavior observed after programming some NAND cells.
EDSFF: A family of ruler and short SSD form factors for dense NVMe deployment in servers and JBOD enclosures.
End-to-end data protection: End-to-end data protection is an integrity scheme whose generation and verification points span an explicitly defined data path.
Enterprise SSD: An enterprise SSD is a solid-state drive intended for server, storage-array or other multi-user storage environments.
Entity tag: An entity tag is an opaque HTTP validator assigned by a server to a selected representation for cache validation and conditional requests.
Erasure coding: Erasure coding encodes data with redundancy so that missing fragments at known locations can be reconstructed from a sufficient, code-dependent set of surviving fragments.
Error-and-erasure decoding: Error-and-erasure decoding reconstructs a codeword while accounting for both known erased positions and incorrect values at unknown positions.
Error-correcting code: An error-correcting code adds structured redundancy to data so a decoder can detect and correct specified error patterns.
errseq_t: errseq_t is a Linux kernel datatype that records an error and sequence state so observers can detect failures since their saved cursor.
ESCON: Earlier IBM optical channel link superseded by FICON for mainframe DASD.
ESDI: Enhanced Small Disk Interface, a faster successor to ST-506 with integrated encoding on the drive.
Ext4 fast commit: Ext4 fast commit is a journaling mechanism that records metadata deltas for eligible updates instead of a full traditional metadata-block commit.
Ext4 orphan file: An ext4 orphan file is an internal metadata file that stores inode-number slots for orphan cleanup when the orphan_file filesystem feature is enabled.
Ext4 orphan inode: An ext4 orphan inode is an inode tracked so recovery can finish deletion or remove excess allocations left by an interrupted truncation or extension.
Extended Asynchronous DRAM Refresh: Extended Asynchronous DRAM Refresh is a platform capability that extends persistent-memory power-fail protection to CPU caches.

F

F1 score: F1 score is the harmonic mean of precision and recall for a specified positive class and decision threshold.
F2FS: F2FS is a Linux log-structured filesystem designed for flash storage devices that expose a block interface.
F2FS checkpoint: An F2FS checkpoint records a consistent filesystem recovery point, including information selecting valid metadata-table versions.
F2FS roll-forward recovery: F2FS roll-forward recovery reconstructs selected post-checkpoint file updates from recovery-marked node blocks.
Fail-slow: Fail-slow describes a component that remains operational but performs more slowly than expected for its operating conditions.
Failure burst: Under an adjacent-gap rule, a failure burst is a maximal sequence of event starts in which every consecutive gap is no greater than a specified window.
Failure domain: A failure domain is a group of components that can be affected by the same failure event because they share a dependency or physical location.
Failure label: A failure label is a recorded outcome assigned to an observation under an explicit rule defining which event counts as failure and when it occurs.
Failure operation ticket: A failure operation ticket is a management-system record of a detected hardware problem and its handling status, rather than necessarily a unique failed component.
False positive rate: The false positive rate is the fraction of observed negative cases incorrectly classified as positive at a specified decision threshold.
FastCDC: FastCDC is a content-defined chunking approach that combines Gear-based boundary selection, sub-minimum skipping and chunk-size normalization.
Fault injection: Fault injection is a testing method that deliberately introduces specified faults into a system to observe detection, propagation and recovery under a defined workload.
FCoE: Fibre Channel over Ethernet tunnels FC frames on lossless Ethernet for converged data center networks.
fdatasync: fdatasync is a file-synchronization operation that persists modified data and the metadata required to retrieve it, without requiring synchronization of all other file metadata.
Fencing token: A fencing token identifies an ownership generation that a receiving service checks to reject operations from an obsolete owner.
Fibre Channel: A high-speed serial fabric used to attach block storage in enterprise SAN environments.
FICON: Fibre Channel attachment for IBM mainframe storage channels.
FIEMAP: A Linux ioctl interface that reports file extent mappings and associated state flags over a requested logical byte range.
File extent: A filesystem mapping for a contiguous range of file blocks and the storage blocks backing that range.
File preallocation: Reserving filesystem allocation for a file range before the application writes that range's data.
File storage: Shared hierarchical files and directories over NFS, SMB, or similar protocols.
Filesystem aging: Filesystem aging is the evolution of allocation layout through file operations that can degrade performance even when the logical contents remain comparable.
Filesystem consistency invariant: A filesystem consistency invariant is a rule relating filesystem metadata that must hold at a defined consistency boundary.
Filesystem freeze: A filesystem freeze temporarily blocks modifications after completing ongoing filesystem transactions and flushing dirty filesystem data, metadata and logs to storage.
Fixed block architecture: SCSI-like fixed sectors adopted on mainframe storage for simplification and FBA volumes.
Flash data retention: Flash data retention is the ability of programmed NAND data to remain recoverable over time without refresh under specified wear, storage and read conditions.
Flash module: A replaceable package of NAND dies and controller used in arrays and custom cards.
Flash translation layer (FTL): A flash translation layer maps host logical addresses to physical flash locations and manages flash-specific operations such as reclamation.
Floating-gate flash: A flash cell design that stores charge on an electrically isolated conductive gate to alter the transistor's threshold voltage.
Floppy disk: Removable flexible magnetic disk media used for software distribution and PC backup before optical and flash.
Fluid dynamic bearing: Spindle motor bearings using lubricant films to reduce vibration in modern HDDs.
Flying height: Flying height is the physical clearance between a flying HDD head/slider region and the disk surface.
Force Unit Access: Force Unit Access is a write-request property requiring the flagged data to reach nonvolatile storage before that request is reported complete.
Forward assembly area: A forward assembly area buffers requested chunks in future restore-output order using advance knowledge from the backup recipe.
Free-space fragmentation: Free-space fragmentation is the division of a filesystem's available allocation space into separated contiguous runs.
fs-verity: fs-verity is a Linux filesystem facility that verifies the immutable contents of selected files against a cryptographic file digest.
fsync: fsync is a file-synchronization operation that requests persistence of a file's modified data and associated metadata before reporting completion.
FTL translation page: An FTL translation page is a flash page containing address-mapping entries.
Full backup: A full backup captures every data object within a defined backup scope, whether or not it has changed since an earlier backup.
Full-page write: A PostgreSQL full-page write records a recoverable page image in WAL to support recovery from an incomplete data-page write.
Full-stripe write: A parity-array write supplying replacement data for every data position of a stripe so its new parity can be computed without reading the prior stripe contents.
Fuzzy checkpoint: A fuzzy checkpoint records recovery information while transaction processing continues, rather than freezing all activity into one instantaneous database image.

G

Garbage collection (flash): Flash garbage collection reclaims a victim erase block by first preserving its valid data elsewhere and then erasing the block for reuse.
Ghost cache entry: A ghost cache entry records the identity or history of an evicted item without retaining its cached payload.
Glass platter substrate: Glass disks replace aluminum on high-capacity platters for better flatness and thermal stability.
Glass storage: Experimental archival storage that encodes data as structural changes inside glass for very long-duration, read-mostly retention.
GMR read head: Giant magnetoresistive heads enabled areal density gains in the late 1990s HDD era.
Gray failure: A gray failure occurs when an application observes a system as unhealthy while that system's failure observer regards it as healthy.
Greedy garbage collection: A flash block-cleaning policy that selects a candidate erase block with the fewest valid pages, minimizing the valid data copied for that selection.
Group commit: Group commit is a logging optimization that shares a durable log synchronization across multiple concurrent transaction or write requests.
Grouped cross-validation: Grouped cross-validation evaluates a predictor using splits that keep all observations from each specified group together in either training or testing within a split.

H

Hard disk drive (HDD): A non-volatile storage device that records data on spinning platters with a moving read/write head.
Hard link: A hard link is a directory entry naming an existing file's inode, so multiple hard links refer to the same file rather than to separate copies.
Hash collision: A hash collision occurs when two distinct inputs produce the same digest under the same hash function and parameters.
Hazard rate: The hazard rate is the instantaneous event rate at a given time conditional on the event not having occurred before that time.
HDD head gas bearing: An HDD head gas bearing is the thin moving gas film whose pressure supports the head-carrying slider against its suspension load.
Head slap: Head slap is a head-disk contact mechanism in which an HDD slider separates from the disk and then snaps back into it.
Head-disk interface (HDI): The head-disk interface is the interacting region between an HDD's head-carrying slider and rotating disk surface.
Heat-assisted magnetic recording (HAMR): An HDD technology that briefly heats the media with a laser so bits can be written on high-coercivity material at higher density.
Helical scan recording: Tape heads scan at an angle across a slow-moving tape, used in DAT and some video formats.
Helium leak port: Helium leak port is an unverified legacy corpus label, not an established name for a lifetime diagnostic interface on helium HDDs.
Helium-filled HDD: A helium-filled HDD is a hard disk drive whose head-disk assembly operates in a sealed helium atmosphere to reduce aerodynamic loading.
Hinted handoff: Hinted handoff retains information about an update missed by an intended replica and attempts to deliver it later.
Hole punching: Deallocating whole filesystem blocks within a file range while preserving the file's logical length and making the affected range read as zeros.
Holographic data storage: Stores pages of data as interference patterns in a photosensitive volume, mostly experimental commercially.
Host bus adapter (HBA): A card or SoC port that connects servers to SCSI, SAS, Fibre Channel, or NVMe fabrics.
Host memory buffer (HMB): Host Memory Buffer is an optional NVMe feature through which a controller uses assigned host-memory ranges.
Host-managed band (SMR): The host tracks which SMR bands are full and schedules sequential writes into empty zones.
Host-managed SMR (HM-SMR): An SMR HDD whose shingled bands are exposed to the host, which must write sequentially within zones.
Host-side flash GC: Host filesystems or FTLs relocate valid pages before erasing flash blocks.
Hot journal: A hot journal is a SQLite rollback journal that requires replay to recover an interrupted database transaction.
Hot spare: A hot spare is a standby device or reserved distributed spare capacity available to take over a failed member's role in a redundant array or pool.
Hot standby feedback: Hot standby feedback is PostgreSQL's mechanism for reporting standby query retention needs upstream to defer conflicting row cleanup.
Hot storage tier: Low-latency media and tiers serving active workloads with frequent reads and writes.
Hot, warm, and cold storage tiers: A data-placement model where hot tiers favor latency, warm tiers favor cost per TB on disk, and cold tiers favor tape or archive media.
Hot/cold data separation: A flash-placement strategy that groups data by update frequency so frequently overwritten pages and long-lived pages need not share the same cleaning pool.
Hybrid flash array: A storage system mixing flash caches or tiers with HDD capacity pools.

I

I/O fencing: I/O fencing enforces exclusion of a node from shared storage so that recovery cannot be undermined by its continued access.
I/O request size: I/O request size is the number of bytes requested by one input/output operation at a specified system boundary.
I/O timeout: An I/O timeout is expiration of a layer's waiting limit for an input/output operation, which may initiate recovery without proving that the operation never executed.
IBM RAMAC: The first commercial moving-head disk storage product, shipped with the IBM 305 RAMAC system.
IDE / ATA: Integrated Drive Electronics places the disk controller on the drive, the ancestor of PATA and SATA.
Idempotency: Idempotency is the property that repeating the same operation with identical inputs has the same intended effect as performing it once.
Idempotency key: An idempotency key identifies attempts of one logical request so a service can recognize retries within its documented scope and retention rules.
INCITS T10 SCSI: The standards body maintaining SCSI command sets used by SAS, FC, and iSCSI block devices.
Incremental backup: An incremental backup records changes relative to an earlier backup state and reuses retained backup data to represent unchanged content.
Independent access range (Linux): A Linux independent access range is an advertised region of a block device that can service requests in parallel with its other reported regions.
Informative censoring: Informative censoring occurs when the observation-ending process conveys information about the event time beyond what the survival analysis accounts for.
Inode: An inode is a filesystem object that holds a file's metadata and has a filesystem-local number, distinct from the directory names and open file descriptions that refer to the file.
Intel Optane: Intel's brand for storage products built on 3D XPoint non-volatile memory before the business was wound down.
Interarrival time: An interarrival time is the elapsed interval between consecutive events in a specified event stream.
Invertible Bloom lookup table: An invertible Bloom lookup table is a randomized data structure that supports updates and can list encoded entries with high probability under its sizing assumptions.
io_uring: io_uring is a Linux asynchronous I/O interface that exchanges requests and completion results through shared submission and completion rings.
io_uring completion queue entry: An io_uring completion queue entry is a result record containing an operation result, flags and application-provided identification data.
io_uring drain: io_uring drain is a submission flag that delays a request until earlier requests in the same ring finish and delays later requests until the drain request finishes.
io_uring integrity metadata: io_uring integrity metadata is a separately described protection-information buffer attached to a read or write request through SQE attributes.
io_uring linked requests: io_uring linked requests are a chain of submission entries whose dependencies delay each successor until its predecessor completes.
IOPS: Observed IOPS is the number of completed input/output operations divided by the measurement duration at a specified system boundary.
iSCSI: SCSI block commands carried over IP networks, allowing SAN-like access without dedicated FC hardware.

J

JBOD: Just a Bunch Of Disks presents individual drives without RAID virtualization in the enclosure.
JEDEC SSD standards: JEDEC publishes endurance and environmental specs referenced by enterprise SSD vendors.
Journal checkpoint: In JBD2 terminology, a journal checkpoint writes committed journaled state to its home locations so the corresponding log space can be reclaimed.
Journal revoke record: A journal revoke record identifies a block whose older journaled images must not be replayed over newer uses of that block.

K

Kaplan-Meier estimator: The Kaplan-Meier estimator is a nonparametric estimator of the survival function that multiplies conditional survival estimates at observed event times while accounting for right-censored observations.
Key-value separation: Key-value separation stores values outside the main key index and retains references in that index.
Key-value SSD: A drive exposing key-value semantics instead of fixed-size LBAs for database and cache offload.

L

Latent sector error: A latent sector error is an inaccessible sector whose failure remains undiscovered until that location is accessed.
Layer-to-layer process variation: Layer-to-layer process variation is fabrication-related variation in cell characteristics between vertical layers of a 3D NAND die.
LDPC (NAND ECC): An LDPC code is an error-correcting linear block code defined by a sparse parity-check matrix.
Least recently used replacement: Least recently used replacement evicts the resident entry whose most recent access is oldest.
Left-truncation: Left-truncation is selection into a time-to-event sample only after a unit has survived to its observation-entry time.
Legal hold (storage): A storage legal hold prevents deletion or replacement of protected data until an authorized action removes the hold, rather than expiring on a preset date.
Leveled compaction: Leveled compaction organizes sorted data into levels with size targets and merges selected data with overlapping key ranges in a destination level.
Linearizability: Linearizability requires concurrent operations on an object to behave as a legal sequential execution that preserves the real-time order of non-overlapping operations.
Linux block-device read-only flag: The Linux block-device read-only flag is a software-controlled device state exposed through block-device ioctls to restrict write access.
Little's law: Little's law relates time-averaged occupancy L to the operation rate lambda and mean residence time W within the same system boundary.
LMDB: LMDB is an embedded transactional key-value database that uses memory-mapped B+ trees, copy-on-write updates and multi-version concurrency control.
LMDB reader table: The LMDB reader table records reader process, thread and transaction identities so page reclamation can respect active snapshots.
Locally repairable code: A locally repairable code is an erasure code that provides recovery of specified missing fragments from a small subset of surviving fragments.
Log sequence number: A log sequence number identifies a position in an engine's ordered recovery-log address space.
Log-likelihood ratio: A log-likelihood ratio is the logarithm of the relative likelihood of two alternatives, used in decoding to express a bit decision and its confidence.
Log-structured merge-tree: A write-oriented index that buffers updates and merges sorted components over time, trading write batching against read and compaction work.
Logical decoding: Logical decoding reconstructs database changes from a storage-level log into a representation that a consumer can interpret without the database's physical storage format.
Logical failover slot: A PostgreSQL logical failover slot is a logical replication slot enabled for synchronization to a physical standby so consumption can resume after promotion.
Logical journaling: Logical journaling records recoverable changes to logical objects or operations rather than simply reproducing their on-disk representation.
Logical sector: A logical sector is the addressable data unit selected by a logical block address at a storage interface.
Logical unit (LUN): A SCSI addressable block device presented by a target to initiators on a SAN.
Longitudinal magnetic recording: Magnetic recording where bit magnetization lies in the plane of the disk, largely replaced by PMR for capacity.
Lost write: A lost write is a write reported as completed whose intended update is absent at the required storage location.
LSM compaction: LSM compaction reorganizes sorted storage components, commonly merging selected runs or SST files and removing obsolete entries when visibility rules permit.
LSM tombstone: An LSM tombstone is a versioned deletion marker that suppresses older values for a key without immediately erasing their stored bytes.
LTFS: Linear Tape File System is a self-describing tape format that stores files and directory metadata for interchange between compatible implementations.
LTFS data placement policy: An LTFS data placement policy specifies which file data may be stored in the index partition alongside filesystem metadata.
LTFS index: An LTFS index is XML metadata describing files, directories and the mapping from file contents to recorded tape extents.
LTO (Linear Tape-Open): An open tape format standard for high-capacity cartridges, advanced by the LTO Program for backup and archive.
LUKS: LUKS is a disk-encryption format that records cryptographic parameters and key-management metadata, including keyslots used to unlock encrypted storage.
LVM: The Linux Logical Volume Manager, a device-mapper framework that builds resizable virtual block devices (logical volumes) out of one or more physical disks or partitions.

M

M-DISC: M-DISC is a family of write-once optical media marketed for archival storage using an inorganic recording layer.
M.2: A small form-factor card standard commonly used for NVMe and SATA SSDs in laptops and desktops.
Magnetic tape storage: Sequential storage on linear magnetic tape in removable cartridges, used mainly for backup and cold archive.
Magneto-optical disc: Optical-magnetic media written with a laser and magnetic field, used for archival and removable storage.
Mainframe DASD: Direct-access storage devices attached to IBM Z systems, historically count-key-data disks.
MAP_SYNC: MAP_SYNC is a Linux mmap flag for supported DAX files that provides a file-offset persistence guarantee used with appropriate CPU persistence instructions.
Maximum distance separable code: A maximum distance separable code achieves minimum distance n - k + 1 for length n and dimension k, allowing recovery from any n - k known symbol erasures.
Mean time to data loss (MTTDL): Mean time to data loss (MTTDL) is the expected time until a storage system first loses data that cannot be retrieved or reconstructed.
Mean time to detection: Mean time to detection is the arithmetic mean of elapsed times between event occurrence and detection for a specified event population.
Medium Auxiliary Memory: Medium Auxiliary Memory is nonvolatile storage associated with an individual storage medium, used to hold auxiliary attributes about the medium and its recorded data.
Memristor storage: Resistive switching devices researched as dense memory; not mainstream datacenter bulk storage.
Memtable: A memtable is an in-memory structure that holds recent LSM-tree updates before they are flushed to sorted table files.
Merkle tree: A Merkle tree combines hashes of data and child nodes into a root so that selected entries or subtrees can be checked without transferring the entire data set.
Metadata update dependency: A metadata update dependency is an ordering requirement between persistent changes needed to preserve a filesystem invariant.
Metal particle tape: Earlier tape coatings using magnetic metal particles before barium ferrite advances.
Microwave-assisted magnetic recording (MAMR): An energy-assisted HDD approach that uses a spin-torque oscillator to lower write coercivity with microwaves instead of laser heating.
Mirror set: A mirror set is a group of storage members configured to maintain copies of the same logical data.
Mirroring (storage): Storage mirroring maintains a copy of source data on another storage device or system.
Misdirected write: A misdirected write stores an update at the wrong address or on the wrong device.
Mission-critical HDD: High-RPM enterprise HDDs for transactional workloads before flash adoption.
MLC SSD: SSDs using two-bit-per-cell NAND, now largely replaced by TLC in consumer lines.
Monotonic reads: Monotonic reads is a session guarantee requiring successive reads to observe states that include the effects of writes relevant to the session's earlier reads.
Monotonic writes: Monotonic writes requires every replica incorporating a later session write to incorporate the session's earlier writes and order them before it.
msync: msync is an operation that synchronizes a range of a shared file-backed memory mapping with its underlying file, with MS_SYNC waiting for completion.
MTBF: Mean time between failures (MTBF) is the expected operating time between successive failures of a repairable system under specified conditions.
Multi-level cell (MLC): NAND flash that stores two bits per cell using four distinguishable threshold-voltage states.
Multipart upload: Multipart upload is an object-transfer process that sends numbered parts independently and assembles them into one object when the upload is completed.
Multipath I/O: Multipath I/O manages multiple paths to the same storage device for failover and, where supported, load distribution.

N

NAND channel: A NAND channel is a communication path between an SSD controller and flash components sharing its transfer resources.
NAND copyback: NAND copyback relocates page data through an on-chip buffer to a destination page without transferring the payload to an external controller.
NAND cross-temperature effect: The NAND cross-temperature effect is the change in read behavior associated with programming and subsequently reading a cell at different temperatures.
NAND die stacking: Stacking multiple flash dies in one package to increase capacity per socket.
NAND erase block: A NAND erase block is a group of flash pages erased together by a block-erase operation.
NAND flash: A non-volatile memory type arranged in NAND strings, forming the storage media in most SSDs and flash cards.
NAND page: A NAND page is a group of data bits addressed together for a page read or program operation within an erase block.
NAND page pairing: NAND page pairing is the grouping of pages whose bits occupy the same physical memory cells.
NAND program interference: NAND program interference is an unintended shift in a cell's threshold voltage caused by programming neighboring cells.
NAND read-reference voltage: A NAND read-reference voltage is a sensing voltage used to distinguish stored cell states during a read operation.
NAND read-retry: NAND read-retry rereads a flash page with adjusted sensing settings after an unsuccessful error-correction attempt.
NAND wordline: A NAND wordline is a shared control-gate connection used to address a group of flash memory cells.
NAS: Network-attached storage serves files over IP to clients without block-level host attachment.
Native Command Queuing: Native Command Queuing (NCQ) allows multiple tagged ATA commands to be outstanding so a supporting device can schedule their execution.
Nearline HDD: A class of high-capacity, lower-RPM hard drives intended for always-on but not primary-tier storage.
Nearline SAS: Lower-RPM SAS drives tuned for capacity and always-on use in servers.
NFS: Network File System is a family of protocols for accessing files and directories on a remote server over a network.
NFS attribute cache: An NFS attribute cache is a client-side copy of file or directory metadata reused until refresh or revalidation is required.
NFS COMMIT: NFSv3 COMMIT requests stable storage of previously buffered writes and associated metadata for a specified file range.
NFS lookup cache: An NFS lookup cache retains directory-name resolution results, including found and missing entries, subject to revalidation.
NFS open-file preservation: NFS open-file preservation is the NFSv4.1 OPEN-result promise to retain a file removed while open, including through the server-restart grace period for open reclamation.
NFS silly rename: NFS silly rename is a client workaround that temporarily renames an in-use file's directory entry and defers its removal until local references are released.
NFS write verifier: An NFSv3 write verifier is a server cookie returned by WRITE and COMMIT that lets clients detect state changes that may have lost uncommitted writes.
NFSv4 file delegation: An NFSv4 file delegation is a recallable server grant allowing a client to manage specified opens and cached file state locally while conflicting access is coordinated by the server.
no_path_retry: no_path_retry is a DM Multipath setting that controls queueing behavior when every path to a mapped device is unavailable.
Noise-predictive maximum-likelihood detection: Noise-predictive maximum-likelihood (NPML) detection incorporates noise prediction or whitening into sequence-detector branch metrics for a partial-response recording channel.
NOR flash: A random-access flash memory type often used for firmware execute-in-place rather than bulk SSD storage.
Normalized magnitude of data loss (NOMDL): Normalized magnitude of data loss (NOMDL) is the expected number of bytes lost per usable terabyte within a stated mission time.
NVDIMM-N: A DIMM form factor combining DRAM with flash backup to persist memory contents across power loss.
NVM subsystem: An NVMe system containing one or more controllers, domains and ports, with zero or more namespaces.
NVMe: A host controller interface and command set optimized for PCIe-attached non-volatile memory such as NAND SSDs.
NVMe Available Spare: NVMe Available Spare is the normalized percentage of spare capacity remaining in the NVM subsystem.
NVMe backplane: Midplane wiring routing PCIe signals from server motherboards to U.2 or EDSFF drive bays.
NVMe base specification: The core NVM Express document defining admin and I/O command sets for PCIe SSDs.
NVMe Composite Temperature: NVMe Composite Temperature is an implementation-specific temperature value in kelvins representing the controller and its associated namespaces in the SMART / Health Information log.
NVMe Controller Busy Time: NVMe Controller Busy Time is the cumulative number of minutes during which the controller has an outstanding I/O command, as defined by the SMART / Health Information log.
NVMe Critical Composite Temperature Time: NVMe Critical Composite Temperature Time is a cumulative SMART / Health log duration in minutes of controller operation with Composite Temperature at or above the critical composite-temperature threshold.
NVMe Critical Warning: NVMe Critical Warning is the SMART / Health Information log bitmask identifying controller warning conditions at the time the log is read.
NVMe Data Units Written: NVMe Data Units Written is the SMART / Health Information log counter of host-written data, excluding metadata, in rounded-up groups of 1,000 units of 512 bytes.
NVMe Device Self-test: NVMe Device Self-test is an optional controller-executed diagnostic capability with short and extended operations whose namespace scope is selected by the initiating command.
NVMe Device Self-test log: The NVMe Device Self-test log is log page 06h, which reports current operation progress and twenty slots for results of completed or aborted self-test operations.
NVMe Endurance Group: An NVMe Endurance Group is a scope within which an NVM subsystem manages endurance.
NVMe Error Information Log Entries: NVMe Error Information Log Entries is the SMART / Health Information log counter of Error Information log entries accumulated over the controller's life.
NVMe FDP: An optional NVMe capability that lets a host guide write placement through placement identifiers while retaining conventional logical-block reads and writes.
NVMe Host Controlled Thermal Management: NVMe Host Controlled Thermal Management is a controller feature through which a host configures temperature targets for automatic transitions to lower-power active power states or vendor-specific thermal actions.
NVMe Media and Data Integrity Errors: NVMe Media and Data Integrity Errors is the SMART / Health Information log counter of unrecovered data-integrity errors detected by the controller.
NVMe multipath: Using multiple routes to the same NVMe namespace for path failover and, where supported, load distribution.
NVMe multipath I/O policy: An NVMe multipath I/O policy is a host-side rule for choosing among eligible routes to one namespace.
NVMe namespace: An NVMe resource collection that a host can address through a namespace identifier, such as formatted non-volatile storage.
NVMe over Fabrics: An extension that carries NVMe commands across RDMA, Fibre Channel, or TCP networks to remote namespaces.
NVMe Percentage Used: NVMe Percentage Used is a vendor-specific estimate of the percentage of NVM endurance consumed by actual use.
NVMe placement identifier: An FDP value combining a reclaim group identifier with a namespace-scoped placement handle that maps to a reclaim unit handle.
NVMe reclaim unit: An FDP storage abstraction within a reclaim group that the controller can physically erase independently of other reclaim units.
NVMe reclaim unit handle: An FDP controller resource that references one reclaim unit in each reclaim group of an endurance group.
NVMe sanitize: NVMe Sanitize starts a controller-managed sanitization operation using a supported Block Erase, Overwrite or Crypto Erase action.
NVMe SMART log: The NVMe SMART log is the standardized log page containing controller health indicators and cumulative usage, endurance and error counters.
NVMe Unexpected Power Losses: NVMe Unexpected Power Losses is the SMART / Health Information log counter of main-power losses that meet the specification's incomplete-shutdown conditions.
NVMe Warning Composite Temperature Time: NVMe Warning Composite Temperature Time is a cumulative health-log duration in minutes governed by composite-temperature thresholds and, in supporting revisions and configurations, temperature hysteresis.
NVMe-MI: NVMe Management Interface for out-of-band monitoring and firmware management of SSDs and enclosures.

O

O_DSYNC: O_DSYNC is an open-file status flag requiring write operations to complete with synchronized I/O data integrity, covering data and the metadata needed to retrieve it.
O_SYNC: O_SYNC is an open-file status flag requiring write operations to complete with synchronized I/O file integrity, covering data and associated file metadata.
Object lock: An object lock enforces retention restrictions on a specified object version through a retention policy or hold.
Object storage: Object storage organizes data as independently addressed objects with associated metadata, accessed through object identifiers or keys rather than a block-device interface.
Object version ID: An object version ID is a service-assigned identifier used with a bucket and key to select an S3 object version.
Object versioning: Object versioning retains separately identifiable revisions of an object under the same key.
Open file description: An open file description is a system-wide open-file object referenced by one or more file descriptors and holding state such as the current file offset and file status flags.
Open-channel SSD: An open-channel SSD exposes media parallelism and placement controls so host software can manage data placement and I/O scheduling.
Open-file-description lock: An open-file-description lock is a byte-range file lock owned by an open file description rather than by a process.
Open-loop load: An open-loop load model schedules request arrivals independently of earlier request completions.
Open-zone limit: An open-zone limit caps the zones simultaneously in implicitly open or explicitly open states.
Optical archival storage: Optical archival storage preserves data on removable laser-readable media as part of a long-term retention and migration workflow.
Optical jukebox: Robotic libraries swapping optical discs for archival retrieval.
Optical-disc lifetime estimate: An optical-disc lifetime estimate predicts time to a defined error threshold for a stated population percentile and storage environment under an aging model.
Over-provisioning: SSD over-provisioning is flash capacity withheld from host-addressable user storage to support controller management such as garbage collection and wear leveling.

P

Page cache: The page cache is the kernel-managed memory that holds cached file contents for access and writeback.
Page LSN: A page LSN records the log sequence number of the latest logged change reflected in a database page under the engine's recovery protocol.
Parity declustering: Parity declustering places parity groups across a larger device pool to distribute reconstruction work.
Parity pollution: Parity pollution is propagation of incorrect data into redundancy information through parity calculations.
Parity RAID: Parity RAID uses computed redundancy across striped data to reconstruct missing chunks without storing complete mirrored copies.
Partial Parity Log: Partial Parity Log is a Linux MD RAID5 mechanism that records the XOR of untouched stripe chunks before dispatching data and parity updates.
Partial quorum: A partial quorum configuration allows a read response set and a write acknowledgement set that need not overlap.
Partition alignment: Partition alignment places a partition's starting offset on boundaries appropriate to the storage topology reported by the underlying stack.
Partitioned filter: An SST membership-filter layout split into independently loadable partitions located through a top-level index.
PATA: A parallel attachment standard for storage devices, dominant in PCs before SATA.
PCIe 4.0 storage: Fourth-generation PCIe roughly doubles per-lane bandwidth used by NVMe SSDs.
PCIe 5.0 storage: Fifth-generation PCIe further increases lane bandwidth for next-wave NVMe drives.
PCIe lanes (storage): A PCIe lane is one pair of transmit and receive signal paths between two PCI Express ports; a link bundles 1, 2, 4, 8 or 16 lanes, so the lane count (x1 to x16) together with the PCIe generation sets the peak bandwidth…
PCIe retimer: A signal conditioner extending PCIe trace length in servers with many NVMe slots.
PCIe storage attachment: Use of PCI Express lanes as the electrical transport for modern high-performance SSDs, usually with NVMe as the protocol.
Peering (Ceph): Ceph peering establishes agreement among OSDs about a placement group's object state and associated metadata.
Perpendicular magnetic recording (PMR): A magnetic recording orientation where bits stand perpendicular to the platter, enabling higher areal density than longitudinal recording.
Perpendicular magnetic tape: Recording orientation on tape analogous to PMR on disks for higher density.
Persistence domain: A persistence domain is the set of components whose stored contents are guaranteed to survive a specified failure under the platform's operating contract.
Persistence ordering: Persistence ordering is a constraint on the order in which updates become durable, rather than merely the order in which an application issues them.
Persistent memory: Persistent memory is nonvolatile storage exposed through a memory-addressable interface, with durability governed by the platform's persistence contract.
Phase-change memory (storage): Memory that switches between amorphous and crystalline states; Intel 3D XPoint marketed a variant.
Physical journaling: Physical journaling records modified on-disk representations, such as metadata buffer contents or changed regions, for recovery.
Physical sector: A physical sector is the device-level data unit whose reported size describes physical read/write granularity rather than logical addressing.
Pipelined Read-Retry: Pipelined Read-Retry overlaps NAND retry sensing with transfer and decoding of earlier attempts.
Placement group (Ceph): In Ceph, a placement group is a logical grouping of objects within a pool that share storage placement and are managed together for replication and recovery.
Planar NAND: Planar (2D) NAND is flash memory in which the cells are built in a single layer on the silicon surface, the layout that 3D NAND replaced as a way to raise bit density.
PLC (penta-level cell): NAND storing five bits per cell; higher density with challenging endurance and read margins.
Point-in-time recovery: Point-in-time recovery restores a database to a selected historical state by applying retained transaction-log history to a suitable backup.
POSIX_FADV_DONTNEED: POSIX_FADV_DONTNEED is a file-access advice value requesting release of cached data that the application does not expect to access soon.
POSIX_FADV_NOREUSE: POSIX_FADV_NOREUSE is a file-access advice value indicating that the application does not expect to reuse the specified data.
posix_fallocate: A POSIX library function that reserves storage for a file byte range and reports failure by returning an error number directly.
Power loss protection: Power-loss protection is a set of hardware and firmware measures intended to preserve specified SSD data and internal state when external power is unexpectedly removed.
Power-on hours: Power-on hours is accumulated powered-operation time reported for a device, interpreted according to that device's counter and interface semantics.
Powersafe overwrite: Powersafe overwrite is the storage property that an interrupted file write leaves bytes outside its requested range unchanged.
Precision: Precision is the fraction of positive predictions that match observed positive outcomes at a specified decision threshold.
Prediction horizon: A prediction horizon is the future interval after a prediction time in which a specified target event is evaluated.
Prefix Bloom filter: A Bloom filter built over extracted key prefixes rather than only whole keys, allowing negative membership tests for those represented prefixes.
Prefix extractor: A key transformation with a defined input domain that produces the prefix used for prefix-based indexing or filtering.
Prefix-preserving crash consistency: A recovery contract in which surviving updates form a prefix of the specified write order, including all writes covered by the last successful flush.
PRML detection: Partial-response maximum-likelihood (PRML) detection recovers a recorded symbol sequence by equalizing readback to a chosen partial-response target and evaluating sequence likelihoods under a channel model.
Process-associated record lock: A process-associated record lock is a byte-range file lock whose ownership is attached to a process rather than to an open file description.
Program/erase cycle: One complete program and erase of a NAND erase block, a core endurance metric.
Protection information (PI): Protection information is integrity metadata accompanying storage I/O, with checks determined by the protocol, protection format and participating layers.
Purge sanitization: Purge sanitization uses suitable logical or physical techniques to make target-data recovery infeasible even with current state-of-the-art laboratory techniques while potentially allowing the medium to be reused.

Q

QLC SSD: Solid-state drives built primarily from quad-level cell NAND for capacity-centric workloads.
Quad-level cell (QLC): NAND flash that stores four bits per cell using sixteen charge levels, optimizing cost per GB over endurance.
Queue depth: Queue depth is the number of outstanding I/O requests at a specified boundary, while a configured depth limit is the maximum or target allowed in flight.
Quorum intersection: Quorum intersection is the property that every relevant pair of allowed quorums shares at least one replica.

R

Racetrack memory: Research memory moving magnetic domain walls along nanowires; not a shipping bulk store.
RAID: Redundant Array of Independent Disks combines multiple drives for performance or fault tolerance.
RAID 10: RAID 10 combines replicated data copies with striping across storage members.
RAID 5: RAID 5 stripes data and distributed single parity across members so that one missing member can be reconstructed when the remaining required information is readable and consistent.
RAID 6: RAID 6 uses dual parity protection to retain data after any two member failures, provided the remaining required data and redundancy are readable and consistent.
RAID rebuild: A RAID rebuild reconstructs the required contents of a replacement or out-of-sync member from the array's surviving redundancy.
RAID tail hour: In The Tail at Store, a RAID tail hour is a group-hour in which at least one data drive has an hourly average I/O latency at least twice its group's median.
RAID write hole: The RAID write hole is the risk that interrupted member updates leave inconsistent parity and corrupt reconstruction of missing data.
RAIDZ: RAIDZ is ZFS's distributed-parity virtual-device layout with one, two or three parity fragments and block-dependent stripe widths.
RAIDZ expansion: RAIDZ expansion widens an existing RAIDZ virtual device by attaching another member and redistributing allocated space while retaining old blocks' data-to-parity ratios.
Range tombstone: A range tombstone is a versioned deletion marker that suppresses covered older keys across an ordered key interval.
Rate of occurrence of failures: The rate of occurrence of failures (ROCOF) is the derivative with respect to time of the expected cumulative failure count of a repairable system.
Rateless IBLT: Rateless IBLT is a set-reconciliation scheme that streams additional coded symbols until the receiver can recover the set difference without first estimating its size.
Raw bit error rate: Raw bit error rate is the number of erroneous bits divided by the total bits read at a specified measurement point before error correction.
RDMA storage networking: Remote direct memory access reduces CPU overhead for NVMe-oF and fast SAN traffic.
Read amplification: Read amplification expresses storage-read work per logical read request or per useful byte returned, under an explicitly stated accounting convention.
Read disturb: Read disturb is an unintended change in NAND cell thresholds caused by reads to other wordlines in the same block.
Read quorum: A read quorum is a protocol-approved set of replicas whose responses satisfy the requirement for completing a read.
Read reclaim: Read reclaim relocates valid NAND data to fresh pages to limit accumulated read-disturb errors.
Read repair: Read repair reconciles divergent replicas as part of processing a read request.
Read-modify-write: A storage update that reads existing data needed to preserve unchanged information, combines it with replacement data, and writes the modified data and any required redundancy.
Read-only mount: A read-only mount restricts filesystem modification through Linux mount state, whose scope can be a VFS mount node, the filesystem superblock, or both.
Read-pass voltage: Read-pass voltage is the bias applied to unselected NAND wordlines to make their cells conduct while another wordline is sensed.
Read-your-writes: Read-your-writes is a session guarantee requiring subsequent reads to use a state that incorporates the session's prior successful writes.
Recall: Recall is the fraction of observed positive outcomes identified by a classifier at a specified decision threshold.
Reconstruct-write: A parity-array update method that computes new parity from incoming replacement data and unchanged data from the same stripe instead of updating old parity from old overwritten data.
Recovery point objective: The recovery point objective specifies the point in time to which data must be recoverable after a disruption, commonly expressed as the maximum tolerable data-loss interval.
Recovery target: A recovery target is the configured condition at which database log replay should stop.
Recovery time objective: The recovery time objective sets the tolerable duration of resource unavailability before its effect on supported processes becomes unacceptable.
Reed–Solomon codes (storage): Reed–Solomon codes are maximum-distance-separable error-correcting codes over finite-field symbols that add n − k check symbols to k data symbols and can rebuild missing (erased) symbols, which is why storage systems…
Reflink: Filesystem support for sharing physical extents between files until one copy is modified.
Repair locality: Repair locality is the number of helper fragments needed to recover a missing fragment under a specified erasure-code repair procedure.
Repair-read amplification: Repair-read amplification is the ratio of bytes read during recovery to the explicitly stated quantity of lost data or lost encoded fragments being restored.
Replay attack: A replay attack reuses previously valid data to induce acceptance in an unauthorized context.
Replication lag: Replication lag is a difference in replication progress between a source and a replica, measured at a named processing stage in a stated unit.
Replication slot: In PostgreSQL, a replication slot tracks a consumer's progress and retains WAL and other resources needed for that consumer to continue.
Resilver: A resilver is ZFS's process of bringing a newly attached, replacement or out-of-date device into agreement with the data it must hold.
Restore test: A restore test exercises recovery from backup material and checks whether the recovered data or selected system functions meet defined acceptance criteria.
Retention interference: Retention interference is dependence of a flash cell's charge-retention behavior on the state of a neighboring cell.
Retry deduplication: Retry deduplication recognizes repeated submissions of one logical request so that an already completed mutation is not executed again.
Ribbon filter: A static approximate-membership filter built using banded Boolean linear systems, offering a space and construction-cost tradeoff relative to Bloom filters.
Right-censoring: Right-censoring records that a time-to-event exceeds the last observed event-free time without observing the event time itself.
Robotic tape loader: Mechanisms that move tape cartridges between slots and drives in a library.
RocksDB atomic flush: RocksDB atomic flush installs the results of flushing selected column families into the database as one logical all-or-nothing MANIFEST group.
RocksDB column family: A RocksDB column family is a separately configurable logical key space within one database, with its own memtables and SST files but a shared write-ahead log.
RocksDB MANIFEST: RocksDB MANIFEST is the version-edit log mechanism used to recover the database's recorded state, with CURRENT identifying the active manifest log.
RocksDB pending compaction bytes: RocksDB pending compaction bytes is an estimate of rewrite work needed to bring LSM levels within their size targets under level-based compaction.
RocksDB write slowdown: A RocksDB write slowdown is the rate-reduction state of write backpressure, distinct from a full write stop.
RocksDB write stall: A RocksDB write stall is backpressure that delays or stops foreground writes when configured resource or background-work limits are reached.
RocksDB WriteBufferManager: RocksDB WriteBufferManager is a shared controller that accounts for memtable memory across participating column families or database instances and can trigger flushing, optional write stalls and block-cache charges.
Rollback journal: A rollback journal is an undo log that preserves original database content so an incomplete transaction can be reversed.
Rollback protection: Rollback protection rejects protected state older than an enforced version or freshness boundary using trusted state that cannot be rolled back with the protected object.
Rolling hash: A rolling hash updates a fingerprint incrementally as the input window or stream advances, instead of recomputing it over the entire input each time.
Rotational latency: Average time for the platter to rotate the requested sector under the head after a seek.
Rotational media: Collective term for HDDs, optical disks, and tape that depend on moving surfaces.
Rotational vibration (RV): Rotational vibration is angular oscillatory motion that can disturb an HDD's ability to keep its heads aligned with recording tracks.
Run-length limited (RLL) encoding: A magnetic recording code limiting consecutive same-polarity bits to increase linear density.
RWF_ATOMIC: RWF_ATOMIC is a Linux pwritev2 flag requesting torn-write protection for the data range of a supported direct-I/O write.
RWF_DONTCACHE: RWF_DONTCACHE is a Linux per-operation flag requesting best-effort removal of page-cache content newly instantiated by a buffered read or write.

S

S.M.A.R.T.: Self-Monitoring, Analysis and Reporting Technology exposes drive health metrics to the host.
SAN: Storage area network provides block access over Fibre Channel or iSCSI to multiple servers.
Sanitization validation: Sanitization validation decides whether to accept or reject a sanitization result after evaluating its evidence, suitability and remaining risk to data confidentiality.
Sanitization verification: Sanitization verification inspects the outcome of a sanitization operation to determine whether the selected technique completed successfully.
SAS expander: A switch-like component that fans out SAS ports so many drives share fewer host links.
SAS PHY event counters: SAS PHY event counters are per-physical-link-endpoint counts of specified link events, including invalid DWORDs, disparity errors, synchronization loss and PHY reset problems.
SATA Express: A connector standard that multiplexed SATA and PCIe lanes for faster client storage before M.2 NVMe.
SATA PHY event counters: SATA PHY event counters record distinct interface events in General Purpose Log 0x11 rather than measuring a disk failure rate.
SCSI: Small Computer System Interface (SCSI) is a family of standards for device commands, logical units, status reporting and the transports that carry them.
SCSI CHECK CONDITION: CHECK CONDITION is a SCSI command status used to report a condition described by sense information.
SCSI initiator and target: In SCSI, the initiator is the endpoint that sends commands and the target is the endpoint that waits for them and performs the requested data transfers; the roles apply across parallel SCSI, Fibre Channel, iSCSI and…
SCSI persistent reservations: SCSI persistent reservations control initiator access to a logical unit using registered keys and a selected reservation type.
SCSI sense key: A SCSI sense key is a four-bit field identifying the broad category of a condition reported in sense data.
SCSI UNMAP: A SCSI command that requests deallocation of specified logical block ranges.
SD Express: Secure Digital cards with a second row of pins for PCIe/NVMe attachment alongside legacy SD modes.
Secure erase: Secure erase is a context-dependent label for a storage erasure procedure whose command, scope and assurance must be specified.
Seek time: Time for the actuator to move the head stack to the correct track on a hard disk.
Self-encrypting drive (SED): A self-encrypting drive encrypts stored data internally using integrated cryptographic and key-management capabilities.
Serial ATA (SATA): A serial bus interface and protocol family for attaching HDDs and SSDs, widely used in PCs and servers.
Serial Attached SCSI (SAS): An enterprise storage interface that extends SCSI semantics over a serial point-to-point or expander topology.
Serializability: Serializability is a transaction-isolation property under which the read results and writes of committed concurrent transactions are equivalent to some serial execution of those transactions.
Session guarantees: Session guarantees are consistency contracts for a session's operations, including read-your-writes, monotonic reads, monotonic writes and writes-follow-reads.
Set reconciliation: Set reconciliation is the task of identifying items present in one of two sets but not the other.
Shingled band: A shingled band is a group of overlapping magnetic tracks separated from neighboring bands by guard regions that limit overwrite propagation.
Shingled magnetic recording (SMR): An HDD recording method that overlaps tracks like roof shingles to raise areal density at the cost of rewrite behavior.
Single-level cell (SLC): NAND flash that stores one bit per cell, trading capacity for endurance and typically faster writes than denser cell types.
SLC cache: A portion of TLC/QLC SSDs programmed in pseudo-SLC mode to absorb bursty writes.
SLC SSD: SSDs built with single-level cell NAND for high endurance industrial or cache use.
Sloppy quorum: A sloppy quorum allows reachable fallback nodes outside a key's normal replica set to participate when intended replicas are unavailable.
Slowdown ratio: In Perseus, the slowdown ratio is observed drive latency divided by the workload-dependent upper prediction bound for that latency.
SMART normalized value: A SMART normalized value is an ATA drive firmware's vendor-specific score for an attribute, interpreted against that attribute's threshold.
SMART raw value: A SMART raw value is the vendor-defined attribute payload reported by an ATA drive, distinct from its normalized score.
smartctl exit status: The smartctl exit status is a process result whose normal ATA interpretation combines independent command, health-threshold and log-history flags in an eight-bit mask.
SMB byte-range lock: An SMB byte-range lock coordinates access to a specified region of a file through shared or exclusive locking associated with a file open.
SMB durable handle: An SMB durable handle allows a client to attempt to recover an existing file open after a network disconnect.
SMB file sharing: Server Message Block is a network protocol family used to access shared files and other server resources.
SMB FLUSH: SMB FLUSH is an SMB2/SMB3 request to propagate cached information for an open file to its backing persistent store, or to wait for a named-pipe reader to consume previously written data.
SMB lease: An SMB lease is a server-granted arrangement that permits selected client-side read, write, or handle caching while coordinating access to a shared object.
SMB opportunistic lock: An SMB opportunistic lock is a revocable server-granted caching arrangement that lets a client reduce network operations while coordinating access to a file.
SMB persistent handle: An SMB persistent handle extends durable-open recovery to server failover on a supporting continuously available share.
SMOTE: SMOTE is an oversampling method that creates synthetic minority-class feature vectors by interpolating between a minority example and selected minority-class nearest neighbors.
SMR garbage collection: SMR garbage collection relocates staged updates and rewrites affected shingled regions to reclaim reusable space.
SMR persistent cache: An SMR persistent cache is a nonvolatile staging area that holds updates before they are incorporated into their target shingled regions.
Snapshot isolation: Snapshot isolation gives a transaction a stable committed snapshot plus its own writes and prevents concurrent transactions that write the same data from both committing.
Snapshot-consistent flash translation layer: A research FTL that recovers a flush-defined disk snapshot instead of an arbitrary later write prefix.
SNIA: Storage Networking Industry Association publishes education and terminology for data infrastructure.
Soft updates: Soft updates is a filesystem metadata-consistency mechanism that delays unsafe on-disk changes until their update dependencies are satisfied.
Soft-decision decoding: Soft-decision decoding uses reliability information about observed symbols rather than only their hard symbol decisions.
Solid-state drive (SSD): A solid-state drive is a storage device that retains data in nonvolatile semiconductor memory without moving recording media.
Solid-state media: Storage without macroscopic moving parts, including NAND, 3D XPoint, and NVRAM.
Sorted string table: An immutable table of sorted key-value entries used as a persistent component in storage engines such as LevelDB and RocksDB.
Space amplification: Database space amplification quantifies storage overhead relative to logical data size under an explicitly stated total-space or excess-space convention.
Sparse file: A file with unallocated ranges that read as zeros, allowing its logical size to exceed its allocated storage.
SQLite super-journal: A SQLite super-journal is a coordination file that links recovery outcomes for a qualifying rollback-mode transaction modifying multiple attached databases on one connection.
SQLite WAL end mark: A SQLite WAL end mark is the committed WAL boundary that determines which page versions a read transaction may observe.
SQLITE_BUSY_SNAPSHOT: SQLITE_BUSY_SNAPSHOT is SQLite's extended result code for a WAL read transaction that cannot become a writer because its snapshot is obsolete.
SSD controller: ASIC or SoC managing NAND channels, ECC, and host protocols on an SSD.
SSD DRAM cache: SSD DRAM is volatile working memory that can hold address mappings, controller metadata or buffered user data, depending on the design.
SSD mapping cache: An SSD mapping cache retains selected logical-to-physical address mappings in memory.
SSD preconditioning: Preparatory writing used to establish a specified SSD state before performance measurement.
SSD steady state: A workload-specific SSD performance condition in which a named metric satisfies stated stability criteria over a defined measurement window.
SSD thermal throttling: SSD thermal throttling is a controller response that limits activity or performance to manage device temperature.
SSHD (hybrid drive): A hard drive with a small NAND cache to accelerate frequently read data while retaining rotating media.
ST-412: Seagate's follow-on full-height 5.25-inch HDD interface companion to the ST-506 ecosystem.
ST-506 interface: An early HDD interface where the drive exposed raw heads and the controller lived on a host adapter card.
Staggered scrubbing: Staggered scrubbing is a scan-order strategy that visits segments across multiple disk regions before returning to subsequent segments in each region.
STATX_WRITE_ATOMIC: STATX_WRITE_ATOMIC is a Linux statx request for a file's reported atomic-write size limits and maximum vector count.
Storage cache flush: A storage cache flush requests persistence of writes buffered in the addressed storage stack, with its coverage and ordering defined by that interface.
Storage immutability: Storage immutability prevents specified data from being overwritten or deleted under an enforced retention rule or hold.
Storage latency: Storage latency is the elapsed time between specified start and completion events for an I/O operation, including any delays inside those measurement boundaries.
Storage snapshot: A storage snapshot is a point-in-time representation of a filesystem or volume, whose implementation may share blocks with its source.
Storage throughput: Storage throughput is the amount of data transferred per unit time at a specified system boundary, usually expressed in bytes per second.
Storage virtualization: A layer abstracts multiple backends into pooled logical volumes presented to hosts.
Storage-class memory (SCM): Media with latency between DRAM and NAND, often byte-addressable, used for persistence tiers.
Striping (storage): Splitting sequential data across drives to aggregate bandwidth.
Submission latency (fio): Submission latency in fio is the elapsed time from initialization of an I/O unit to an engine-dependent submission boundary.
Succinct range filter: An approximate point-and-range membership structure based on a truncated order-preserving Fast Succinct Trie.
Superparamagnetic limit: The grain-size floor where longitudinal bits become thermally unstable, motivating PMR and energy-assisted recording.
Survival function: The survival function S(t) = P(T > t) is the probability that a time T to a defined event exceeds t, measured from a specified time origin.
Symbol erasure: A symbol erasure is a codeword position whose value is unavailable or deliberately discarded and whose location is known to the decoder.
sync_file_range: sync_file_range is a Linux-specific operation that initiates or waits for page-cache writeback over a file range without synchronizing file metadata or flushing volatile device write caches.
syncfs: syncfs is a Linux system call that synchronizes pending file data and metadata for the filesystem selected by an open file descriptor.
Synchronous replication: Synchronous replication makes successful completion depend on acknowledgement that a specified set of replicas has reached the processing stage required by the replication contract.
Synthetic full backup: A synthetic full backup reconstructs a complete backup for a selected recovery point from retained full and incremental backup data without rereading the entire protected dataset from production.
Systematic code: A systematic code includes the original data symbols unchanged among its encoded symbols.

T

Tail latency: Tail latency describes the high-latency part of an operation response-time distribution, usually summarized using specified high percentiles.
Tape buffer synchronization: Tape buffer synchronization is completion of a supported operation that confirms previously buffered tape data and marks have been written to the medium.
Tape encryption: Encrypting data as it is written to tape cartridges to protect offline media.
Tape filemark: A tape filemark is a recorded logical delimiter separating files in a sequential tape stream.
Tape library: Automated systems inserting LTO or enterprise tape cartridges into drives.
TapeAlert: TapeAlert is a SCSI status-reporting mechanism for tape devices that exposes condition flags through LOG SENSE page 2Eh.
TBW: Terabytes Written rating estimates total host writes an SSD warranty covers before wear exhaustion.
TCG Opal: A Trusted Computing Group spec for managing hardware encryption on self-encrypting storage devices.
Temporal validation: Temporal validation evaluates a predictor on later observations using training information available before the evaluation boundary.
Thermal flying-height control (TFC): Thermal flying-height control uses an embedded heater to adjust HDD head-disk clearance through local thermal protrusion.
Thin provisioning: Presenting logical storage capacity while allocating physical backing from a shared pool as writes require it.
Thunderbolt storage: PCIe tunneling over Thunderbolt enables external NVMe enclosures at near-internal link speeds.
Tiered compaction: Tiered compaction allows multiple sorted runs to accumulate and merges groups of runs into typically larger runs instead of maintaining one run at every tier.
Tiled magnetic recording: A proposed patterning approach grouping grains into magnetic tiles for higher density.
TLC SSD: SSDs using triple-level cell NAND as the mainstream balance of cost and endurance.
TMR read head: Tunnel magnetoresistance sensors succeeded GMR heads for higher signal on dense tracks.
Torn write: A torn write is an interrupted write that persists only part of its intended update, potentially leaving a database page with mixed old and new contents.
Touchdown sensor (TDS): An HDD touchdown sensor is a temperature-sensitive element used to observe thermal changes near the head-disk interface during clearance and contact studies.
TRIM: The ATA DATA SET MANAGEMENT Trim function tells a device that specified logical block ranges are no longer needed.
Triple-level cell (TLC): NAND flash that stores three bits per cell using eight charge levels, common in consumer and many enterprise SSDs.

U

U.2 / U.3: Enterprise SSD bay and connector family derived from SFF-8639, supporting NVMe and legacy SAS/SATA pinouts in U.3.
UASP: USB Attached SCSI Protocol (UASP) transports SCSI commands over USB and supports multiple outstanding commands, without by itself guaranteeing throughput, SMART pass-through or durable writes.
UDO: Ultra Density Optical was a 30 GB-class professional WORM optical cartridge format.
Uncorrectable bit error rate: Uncorrectable bit error rate is a storage reliability metric that normalizes unrecovered data-error outcomes by a specified bit count after the stated correction and recovery procedures.
Unlink: Unlink is the removal of a directory entry naming a file; POSIX semantics preserve existing open references, while network-filesystem behavior requires additional qualification.
UNMAP: The SCSI UNMAP command requests deallocation of logical blocks.
Unserializable write: An unserializable write is a storage failure in which recovered contents cannot be reconciled with an operation order allowed by the known write history.
Unwritten extent: An allocated filesystem extent whose data is not initialized and is exposed as zeros when read through the filesystem.
Up set (Ceph): In Ceph, the up set is the ordered OSD list assigned to a placement group by CRUSH for a particular map epoch.
USB mass storage: The USB device class that presents block storage to hosts using BOT or UASP transports.

V

V-NAND: Samsung's brand for its charge-trap 3D NAND vertical stacking architecture.
Value log: A value log is an append-oriented record store whose values are located through a separate index.
Volume manager: Host or array software that groups physical drives into logical volumes and snapshots.

W

WAL archiving: WAL archiving preserves write-ahead-log segments for later recovery before normal log recycling can remove them.
Warm storage tier: Capacity-oriented online storage balancing cost and occasional access latency.
Wear leveling: A flash management technique that spreads erase cycles across blocks so no single region wears out prematurely.
Winchester disk architecture: Winchester disk architecture combines low-mass, lightly loaded flying heads with lubricated disks in a protected head-disk assembly; it does not require non-removable media.
WORM storage: Write-once-read-many storage preserves recorded content against subsequent alteration while allowing repeated reads.
Write amplification: Device-level byte write amplification is the ratio of bytes programmed to the storage medium to bytes written by the host over the same accounting interval.
Write combining: Merging adjacent small writes into fewer large operations to reduce amplification on flash or SMR.
Write quorum: A write quorum is a protocol-approved set of replicas whose acknowledgements satisfy the requirement for completing a write.
Write skew: Write skew is a concurrency anomaly in which transactions read overlapping data and make disjoint writes whose combined outcome can violate an invariant that each transaction would preserve alone.
Write-ahead log: A write-ahead log is a recovery log whose relevant records must become durable before the corresponding changes are persisted in database data files.
Write-back cache: Write-back caching completes a write after updating the cache and defers updating its backing store until later.
Write-intent bitmap: A write-intent bitmap tracks regions of a redundant array that may need synchronization after interrupted writes.
Write-once media: Storage that physically or logically prevents rewriting committed regions.
Write-through cache: Write-through caching propagates writes to the backing store as part of completing them at the cache layer.
Writeback error: A writeback error is a failure encountered while transferring modified cached data to its backing storage.
Writes-follow-reads: Writes-follow-reads requires every replica incorporating a session write to incorporate and order before it the updates relevant to that session's earlier reads.

X

XFS Active Item List: The XFS Active Item List tracks logged items whose journal records remain needed until later retirement.
XFS Committed Item List: The XFS Committed Item List tracks in-memory logged changes awaiting a journal checkpoint.
XFS deferred operation: An XFS deferred operation postpones metadata work to a later transaction to satisfy locking or reservation constraints.
XFS delayed logging: XFS delayed logging aggregates metadata changes in memory before committing them to the filesystem journal.
XFS intent log item: An XFS intent log item describes deferred work that log recovery may need to complete.
XFS relogging: XFS relogging records an object's outstanding metadata changes again in a newer transaction.
XFS rolling transaction: An XFS rolling transaction commits one step of a transaction chain and continues with a fresh transaction using a permanent log reservation.
XTS-AES: XTS-AES is a tweakable AES encryption mode for storage data units that provides confidentiality without authenticating the data or its source.

Z

ZAC: Zoned ATA Commands are the SATA counterpart to ZBC for host-managed SMR HDDs.
ZBC: Zoned Block Commands standardize host-managed SMR access for SCSI/SATA devices.
ZFS error scrub: A ZFS error scrub is an integrity scan restricted to files already recorded with data errors.
ZFS Intent Log: The ZFS Intent Log is a per-dataset recovery log that can persist filesystem operations before their transaction group reaches stable storage.
ZFS separate log: A ZFS separate log is dedicated pool log-device storage used for intent-log blocks instead of their default allocation in the main pool.
ZFS transaction group: A ZFS transaction group is an ordered batch of changes whose synchronization advances the pool's consistent persistent state.
Zip drive: A removable cartridge drive format from Iomega offering tens to hundreds of megabytes per disk in the 1990s.
Zone Append: The native NVMe Zone Append command writes at a controller-selected position within a specified zone and returns the assigned starting logical block address on successful completion.
Zone capacity: Zone capacity is the maximum number of logical blocks that may be written within a zone.
Zone Finish: Zone Finish changes an eligible sequential zone to Full, ending further writes until reset without discarding the data already written.
Zone Reset: Zone Reset makes an eligible sequential zone empty for reuse, returns its write pointer to the zone start and invalidates access to its previous data.
Zone write locking: A Linux mechanism that serializes covered write requests to the same sequential zone using a per-zone lock.
Zone write plugging: A Linux block-layer mechanism that queues covered writes per sequential zone before dispatch to preserve the issuer's sequential write pattern.
Zone write pointer: A zone write pointer identifies the next writable logical block address in a sequential zone when its current state makes that pointer valid.
Zoned block device (ZBD): A zoned block device exposes a logical block address space divided into zones with zone-specific access rules.
Zoned namespace: A zoned namespace is an NVMe namespace whose logical address space follows the ZNS command set's zone organization and write constraints.
Zoned namespaces (ZNS): Zoned Namespaces (ZNS) is an NVMe command set that organizes a namespace into zones with sequential-write constraints and explicit reset before reuse.