Deterministic accounting / no device measurements

RAIDZ expansion: ideal retained parity overhead

Five deterministic mixtures of old and new full-width parity geometry for the same 6 TiB payload. Not measured storage capacity or ZFS space-report output.

JSON and provenance / Model code / Generator / Research explanation

Method

Hold payload D=6 TiB and parity P=2 fixed; old width N=4, new width M=5. Enumerate old payload fractions f=0,0.25,0.5,0.75,1. Compute raw=D*f*N/(N-P)+D*(1-f)*M/(M-P); extra=D*f*P*(M-N)/((N-P)*(M-P)). TiB means 2^40 bytes. CSV rounds numbers to nine decimal places; the in-page table uses these exact half-TiB example values. Each row is an independent hypothetical cohort mixture, not an observed time step.

Reproduce

Run beside the model file with Node.js; no packages, device access or random seed are required. The output is byte-identical CSV. The generator also writes JSON and SHA-256 hashes; generatedAt is a build timestamp, not an observation period.

node --input-type=module -e 'import { toCsv } from "./raidz-expansion.mjs"; process.stdout.write(toCsv())'

For the 50% row, three TiB retain the 2:2 data-to-parity ratio and three TiB use 3:2. The resulting 11 TiB is one TiB above the all-new counterfactual. That counterfactual assumes the old allocations are freed.

Limits

• This is deterministic algebra, not a hardware benchmark, field dataset or replication of FastScale; no observation period, sampling interval or statistical confidence interval applies.
• Assumes full-width equal-size fragments, fixed payload and unchanged parity. Real RAIDZ blocks can use variable widths and incur allocation padding.
• Excludes metadata, compression, deduplication, snapshots, clones, reservations, free-space headroom and filesystem accounting conventions.
• The all-new counterfactual assumes old blocks are freed. Retained snapshot or clone references can make a rewrite consume additional space.
• Multiple historical expansion widths require separate cohort accounting; this export contains only one old and one new geometry.
• No prediction of rewrite duration, error rate, recoverability, actual usable capacity or reported AVAIL.

Column definitions

oldWidth

Original physical width, including parity; 4 in this example.

newWidth

Final physical width, including parity; 5 in this example.

parity

Unchanged parity-fragment count per full row; 2.

payloadTiB

Total payload in TiB (2^40 bytes), excluding parity; 6.

oldFraction

Fraction of payload bytes retaining the old data-to-parity ratio, not pool occupancy.

oldPayloadTiB

Payload at the old ratio, in TiB.

newPayloadTiB

Payload at the new ratio, in TiB.

oldParityTiB

Ideal parity allocated for the old cohort, in TiB.

newParityTiB

Ideal parity allocated for the new cohort, in TiB.

rawTiB

Modeled total payload plus parity, in TiB; not zfs list or zpool list output.

allNewRawTiB

Counterfactual allocation if all payload used the new ratio and old blocks were freed, in TiB.

retainedExtraTiB

Extra allocation relative to that all-new counterfactual, in TiB; not promised reclaimable space.

Sources and license

CC0-1.0 for Hesela's original model and generated data; cited sources retain their own rights.

1. OpenZFS 2.3: zpool-attach(8), RAID-Z expansion and space reporting (accessed 2026-10-16)
2. OpenZFS zfs-2.3.4: zfs-rewrite(8), snapshot and clone space consequences (accessed 2026-10-16)
3. OpenZFS zfs-2.3.0: zpoolconcepts(7), RAID-Z virtual devices (accessed 2026-10-16)

Definitions: RAIDZ and RAIDZ expansion. Compare the separate repair-placement model; it counts helper reads, not capacity.