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ZFS Calculator

Enterprise Resource Center • Storage Calculator

ZFS RAIDZ Capacity Calculator

Estimate raw and usable storage capacity for ZFS mirrors, RAIDZ1, RAIDZ2, RAIDZ3 and striped top-level vdev configurations. Compare drive count, vdev width, redundancy overhead and recommended operating headroom before building your storage pool.

Mirrors RAIDZ1 RAIDZ2 RAIDZ3 Multi-VDEV Pools
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Pool Configuration

Configure Your ZFS Pool

Drives are divided equally among the top-level VDEVs.
20%
Optional planning headroom. This is not intended to model every internal ZFS reservation or allocation behavior.
Estimated Results

RAIDZ2 Pool

8 drives • 1 VDEV
Approximate ZFS Capacity
72 TB
≈ 65.48 TiB before operating reserve
RAW CAPACITY
96 TB 87.31 TiB
AFTER RESERVE
57.6 TB 52.38 TiB
REDUNDANCY OVERHEAD
24 TB 25% of raw capacity
CAPACITY EFFICIENCY
75% Before free-space reserve
1
TOP-LEVEL VDEV
8
DRIVES / VDEV
2
FAILURES / VDEV
Capacity Model
1 × (8 − 2) × 12 TB = 72 TB
Pool Layout

Visualize Your ZFS VDEVs

Each box below represents one top-level vdev. Redundancy is provided inside the individual vdev; ZFS then distributes data across the top-level vdevs in the pool.

Understanding the Results

How the ZFS Calculator Estimates Capacity

For RAIDZ, this calculator uses the standard planning estimate of the number of data drives in each vdev multiplied by the capacity of the smallest drive. RAIDZ1 uses one parity equivalent, RAIDZ2 uses two and RAIDZ3 uses three.

RAIDZ1
(N − 1) × Drive Size Single parity with one drive-equivalent of capacity used for redundancy.
RAIDZ2
(N − 2) × Drive Size Double parity with two drive-equivalents used for redundancy.
RAIDZ3
(N − 3) × Drive Size Triple parity with three drive-equivalents used for redundancy.
MIRROR
1 × Drive Size / Mirror Each mirror vdev stores one usable copy regardless of whether it contains two or three identical members.
Capacity Units

Why Does a 12 TB Drive Not Show as 12 TiB?

DECIMAL

TB / GB

Drive manufacturers normally advertise capacity using decimal units. One terabyte equals 1,000,000,000,000 bytes.

BINARY

TiB / GiB

Binary capacity uses powers of 1024. One tebibyte equals 1,099,511,627,776 bytes, so the numeric TiB figure is lower than the advertised TB figure.

Important

This Is a Planning Calculator, Not an Exact ZFS Space Predictor

Actual usable ZFS filesystem capacity can differ from this estimate. RAIDZ allocation depends on factors including vdev width, ashift, record size, metadata, block allocation, compression, snapshots, dataset properties and the workload's I/O pattern.

The calculator is therefore best used to compare pool layouts and estimate raw capacity, redundancy cost and operating headroom before deployment.

ZFS Architecture

Fault Tolerance Applies Per VDEV

A pool containing multiple RAIDZ or mirror vdevs does not combine their redundancy into one larger fault-tolerance number. Each top-level vdev must remain viable for the pool to remain viable.

EXAMPLE
2 × 6-Drive RAIDZ2 Each six-drive RAIDZ2 vdev can tolerate two device failures within that particular vdev.
DO NOT ASSUME
“Four Drives Can Fail Anywhere” Failure distribution matters. Losing more drives than a particular vdev can tolerate can make that vdev—and therefore the pool—unavailable.
Frequently Asked Questions

ZFS Capacity Calculator FAQ

How is RAIDZ2 capacity calculated?

A useful raw-capacity estimate is the number of drives in the RAIDZ2 vdev minus two parity equivalents, multiplied by the capacity of the smallest drive.

How much capacity does RAIDZ3 lose?

RAIDZ3 uses approximately three drive-equivalents of each vdev for triple-parity protection.

Can I calculate multiple RAIDZ vdevs?

Yes. Select the total number of drives and the number of equal top-level vdevs. The calculator divides the drives evenly among them.

Why does this calculator show TB and TiB?

Drive manufacturers normally use decimal TB, while binary TiB uses powers of 1024. Showing both makes the apparent capacity difference easier to understand.

Is calculated RAIDZ capacity exact?

No. Actual ZFS space efficiency also depends on block size, sector size, allocation, metadata and other filesystem behavior.

Why include a free-space reserve?

It provides a planning target for keeping some pool capacity unused rather than designing around the theoretical maximum usable capacity.

Does ZFS combine redundancy across vdevs?

No. Redundancy is provided within each top-level mirror or RAIDZ vdev. The pool then distributes data across its top-level vdevs.

Does RAIDZ replace backups?

No. RAIDZ provides storage redundancy, not an independent backup copy of important data.

Enterprise ZFS Storage

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Tech Supply Direct can help identify compatible enterprise HDDs, SSDs, NVMe drives, HBAs, ECC memory and server hardware for ZFS storage systems, virtualization hosts and capacity upgrades.