QNAP 4x 20TB RAID-Z2 NAS Calculator

Estimate usable TB, parity, and fault tolerance for QNAP NAS users using 4x 20TB in RAID-Z2.

Capacity Snapshot

Raw Capacity

80.00 TB

Usable Capacity

36.00 TB

Fault Tolerance

2 drives

Efficiency

50.0%

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	54.00 TB	1 drive	75.0%
RAID 6	36.00 TB	2 drives	50.0%
RAID 10	36.00 TB	1 drive per mirror pair*	50.0%
RAID-Z1	54.00 TB	1 drive	75.0%
RAID-Z2	36.00 TB	2 drives	50.0%

QNAP Planning Notes

QNAP buyers often compare capacity efficiency against service consolidation needs, where VM storage, containers, and media serving all compete for IOPS and free space.

• Model around mixed workloads: media throughput plus VM random I/O.
• Reserve enough headroom so thin-provisioned services do not overrun the array.
• When in doubt, prioritize safer parity for arrays above 6 drives.

Brand / Region Glossary

Qtier

QNAP auto-tiering system that moves hot and cold data across storage classes.

Storage & Snapshots

QTS module for RAID pool management, volume control, and snapshots.

Thin Provisioning

Allocates logical space first and consumes physical capacity over time.

NAS Cluster Guides

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

Related Long-Tail Calculators

2x 4TB RAID 1 2x 6TB RAID 1 2x 8TB RAID 1 2x 10TB RAID 1 2x 12TB RAID 1 2x 14TB RAID 1 2x 16TB RAID 1 2x 18TB RAID 1 2x 20TB RAID 1 3x 4TB RAID 1 3x 4TB RAID 5 3x 4TB RAID-Z1 3x 6TB RAID 1 3x 6TB RAID 5 3x 6TB RAID-Z1 3x 8TB RAID 1

Sequential Long-Tail Navigation

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FAQ

Should I optimize this 4-drive plan for capacity 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.

How much usable storage does 4x 20TB RAID-Z2 provide?

For QNAP users, this NAS planning scenario estimates 36.00 TB usable after a 10% reserve from 80.00 TB raw.

Why include a 10% reserve when planning NAS available space?

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

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.