Unraid 8x 20TB RAID-Z2 NAS Calculator

Estimate usable TB, parity, and fault tolerance for Unraid NAS users using 8x 20TB in RAID-Z2.

Capacity Snapshot

Raw Capacity

160.00 TB

Usable Capacity

108.00 TB

Fault Tolerance

2 drives

Efficiency

75.0%

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	126.00 TB	1 drive	87.5%
RAID 6	108.00 TB	2 drives	75.0%
RAID 10	72.00 TB	1 drive per mirror pair*	50.0%
RAID-Z1	126.00 TB	1 drive	87.5%
RAID-Z2	108.00 TB	2 drives	75.0%

Unraid Planning Notes

Unraid users often optimize for incremental growth and flexible disk replacement, where parity planning and usable capacity targets evolve over time rather than all at once.

• Capacity plans should include phased drive upgrades, not only day-one layout.
• Use parity-protected growth steps to avoid risky bulk migrations.
• Track cache and parity pressure separately from archive data growth.

Brand / Region Glossary

Parity Disk

Dedicated parity disk(s) protecting data disks in the Unraid array model.

Cache Pool

Fast SSD/NVMe pool used for writes and application workloads.

Mover

Scheduled process that migrates data between cache and array tiers.

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

Is RAID-Z2 still practical 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.

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.

How much real-world usable storage does 8x 20TB RAID-Z2 provide?

For Unraid users, this NAS planning scenario estimates 108.00 TB usable after a 10% reserve from 160.00 TB raw.