TrueNAS 10x 20TB RAID-Z2 NAS Calculator

Estimate usable TB, parity, and fault tolerance for TrueNAS NAS users using 10x 20TB in RAID-Z2.

Capacity Snapshot

Raw Capacity

200.00 TB

Usable Capacity

144.00 TB

Fault Tolerance

2 drives

Efficiency

80.0%

Popular TrueNAS default for medium arrays; dual-parity with good safety margin. This scenario applies a 10% filesystem reserve.

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Alternative Mode Comparison

Mode	Usable	Tolerance	Efficiency
RAID 5	162.00 TB	1 drive	90.0%
RAID 6	144.00 TB	2 drives	80.0%
RAID 10	90.00 TB	1 drive per mirror pair*	50.0%
RAID-Z1	162.00 TB	1 drive	90.0%
RAID-Z2	144.00 TB	2 drives	80.0%

TrueNAS / ZFS Planning Notes

TrueNAS and ZFS planners usually care about parity width, scrub cadence, and healthy operating headroom. Capacity is only one part of pool durability.

• Treat RAID-Z2 as a common baseline for medium and large homelab pools.
• Keep meaningful free space to reduce fragmentation pressure under snapshots.
• Plan vdev growth strategy early, since expansion paths affect long-term efficiency.

Brand / Region Glossary

vdev

A virtual device group in ZFS; multiple vdevs form a storage pool.

Scrub

Background integrity scan that verifies checksums and repairs parity mismatches.

RAID-Z Expansion

ZFS feature set and planning topic for growing parity groups safely.

NAS Cluster Guides

NAS Hub RAID Level Compare RAID Capacity Matrix Planning Quick Sheet RAID Selector NAS FAQ

Related Long-Tail Calculators

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Sequential Long-Tail Navigation

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FAQ

How many disk failures can RAID-Z2 tolerate in this setup?

This setup can tolerate 2 drives. Real-world survivability depends on mirror placement, rebuild stress, and drive health.

Why include a 10% reserve when planning NAS storage headroom?

Keeping free space improves filesystem behavior for snapshots, metadata, and write performance. Full arrays often perform worse and rebuild more slowly.

Should I optimize this 10-drive plan for available space or resiliency first?

For long-lived NAS pools, resiliency first is usually safer. Capacity can be expanded later, while a risky parity choice can force migration sooner.

Is RAID-Z2 still worth deploying with 20TB drives?

It can be practical, but larger drives increase rebuild windows. Validate parity choice and backup policy before committing to the final layout.