TrueNAS NAS Storage: ZFS Capacity, RAIDZ, and Real Usable Space
TrueNAS uses ZFS as its storage filesystem, which adds complexity beyond standard RAID math. The 2 GiB swap-per-drive deduction is vendor-published; the wider ZFS overhead varies significantly by vdev layout, pool configuration, and utilisation.
See exactly how much usable storage your configuration delivers.
TrueNAS and ZFS
TrueNAS CORE and SCALE both use OpenZFS as the filesystem and volume manager. ZFS integrates RAID functionality (as RAIDZ), filesystem, and volume management into one layer. Unlike Linux mdraid systems where these are separate.
ZFS is more capable than traditional RAID but adds overhead that is harder to predict precisely. The RAID calculator applies the documented 2 GiB swap-per-drive deduction and a 3.1% ZFS metadata estimate. Exact Mode is recommended for TrueNAS configurations.
RAIDZ Types
- RAIDZ1 (≈ RAID 5): One drive of parity per vdev. (n−1) drives usable. Minimum 3 drives per vdev.
- RAIDZ2 (≈ RAID 6): Two drives of parity per vdev. (n−2) drives usable. Minimum 4 drives per vdev.
- RAIDZ3: Three drives of parity per vdev. (n−3) drives usable. Minimum 5 drives per vdev. Rarely used in home setups.
- Mirror: Two drives. One drive of usable capacity. Equivalent to RAID 1. Multiple mirrors can be combined into a mirrored pool.
- Stripe: No redundancy. Full capacity. Not recommended for data storage. Equivalent to RAID 0.
ZFS Overhead: What the Calculator Applies
Two documented overhead sources are applied by default in Quick Mode:
- Swap partitions: TrueNAS creates a 2 GiB swap partition on each drive in the pool. This is a vendor-published figure from TrueNAS documentation.
- ZFS metadata (~3.1%): The ZFS SPA (Storage Pool Allocator) reserves a "slop space" of approximately 1/32 of total pool capacity (3.1%), capped at 128 GiB. This prevents the pool from filling completely, which would cause ZFS to become read-only.
Additional ZFS overhead (metaslab alignment, indirect block overhead, record size padding) varies by workload and configuration. For precise capacity planning, use Exact Mode and enter the values your TrueNAS pool actually reports.
ZFS Pool Capacity Rule of 80%
ZFS performance degrades when a pool exceeds approximately 80% utilisation. This is due to fragmentation of the copy-on-write block structure. The RAID calculator's headroom slider defaults to 10% but we recommend setting it to 20% for TrueNAS pools.
Frequently Asked Questions
Why is TrueNAS capacity harder to calculate than other brands?
ZFS integrates multiple layers. RAID, filesystem, and volume manager. Each with its own overhead. Metaslab alignment can waste significant space depending on vdev size and recordsize. The jro.io OpenZFS Capacity Calculator provides more detailed ZFS-specific calculations for advanced users.
What is the difference between TrueNAS CORE and SCALE?
Both use the same ZFS storage stack and RAID calculator applies equally to both. SCALE runs on Linux (vs FreeBSD for CORE) and supports additional virtualisation features, but the storage capacity calculations are identical.
Can I expand a RAIDZ pool by adding drives?
Traditional RAIDZ vdevs cannot be expanded by adding drives after creation. You must add a new vdev to the pool. OpenZFS 2.2+ (available in TrueNAS SCALE 24.10+) introduced RAIDZ expansion, but this is a relatively new feature. Check your version before planning expansions.