hesela.devDirect-attached storage
Direct-attached storage (DAS) is storage connected locally to, and dedicated to, a server.
DAS describes the server-to-storage attachment, not a recording medium or a file-sharing protocol. The local connection can lead to an external SAS enclosure through a host bus adapter or RAID controller; direct attachment does not mean that the drives must be inside the server. [1] [4]
NAS describes file access offered to network clients. A server can read locally attached disks while exporting their file system through NFS or SMB. Those are different interfaces in the same installation; a NAS server can also use other backing storage. A SAN is a storage-focused network, commonly but not necessarily providing block services. [2] [5] [3]
The DAS label alone does not determine host count at enclosure level. Documented SAS expansion configurations include multiple directly attached hosts, with zoning to partition drives or cluster software to coordinate shared access. The cited enclosure guide allows unzoned multi-host sharing with HBAs, not RAID controllers. Check which devices are assigned to which hosts rather than treating a shelf as one indivisible device. [4]
Shared-device visibility does not coordinate file-system caches. Cluster file systems add coordination above block access. In Frangipani's historical design, a writer flushed dirty blocks before handing off its lock, while releasing a lock invalidated the associated cache entry. This illustrates a separate software responsibility, not a guarantee supplied by SAS. [5] [6]
Relations
Advantages
- External drive expansion can use a compatible SAS HBA or RAID adapter without adding a network file-service interface to that attachment. [4]
Limitations
- In the documented SAS configurations, multiple paths are managed by RAID-controller firmware or host multipathing software. [4]
Citations
- SNIA dictionary: Direct Attached Storage
- SNIA dictionary: Network Attached Storage
- SNIA dictionary: Storage Area Network
- Lenovo Press LP0512: D1212 and D1224 enclosures, host connectivity (withdrawn products)
- Red Hat Enterprise Linux 10: Overview of available file systems, local and network file systems
- Thekkath, Mann and Lee, Frangipani: A Scalable Distributed File System, SOSP 1997, sections 2.1, 5 and 6
Sources & caveats
- The definition follows the current SNIA device-level meaning; enclosure documentation uses DAS more broadly for supported multi-host configurations. [1] [4]
- LP0512 describes withdrawn Lenovo D1212/D1224 products and is an architecture example, not a product recommendation or a universal host-count limit. [4]