RAIDZ3
What Is RAIDZ3?
RAIDZ3 is a ZFS RAIDZ layout that uses triple parity. A RAIDZ3 VDEV can tolerate up to three unavailable member devices without exceeding its parity protection.
The additional parity provides greater fault tolerance than RAIDZ1 or RAIDZ2, but it also consumes more raw capacity. RAIDZ3 is therefore most attractive when protecting data is more important than maximizing usable capacity.
RAIDZ1 → RAIDZ2 → RAIDZ3
The three standard RAIDZ levels are primarily distinguished by the amount of parity protection provided within each VDEV.
RAIDZ1
Tolerates up to one member-device failure within the VDEV.
RAIDZ2
Tolerates up to two member-device failures within the VDEV.
RAIDZ3
Tolerates up to three member-device failures within the VDEV.
How Does RAIDZ3 Work?
RAIDZ3 distributes data and parity information across the devices participating in the VDEV. Three parity levels provide enough redundancy to reconstruct missing information when as many as three member devices are unavailable.
RAIDZ3 Can Tolerate Three Device Failures
A healthy RAIDZ3 VDEV can lose one, two or three member devices and still retain enough redundancy to reconstruct the missing information.
RAIDZ3 Usable Capacity
In a simplified equal-drive configuration, RAIDZ3 uses approximately three drives' worth of raw capacity for parity.
N represents the number of equal-size member devices. Actual usable capacity is lower after filesystem, metadata, allocation, unit-conversion and operational free-space considerations.
RAIDZ3 Capacity Examples
| Drives | Drive Size | Raw Capacity | Simplified RAIDZ3 Capacity | Failures Tolerated |
|---|---|---|---|---|
| 5 | 8 TB | 40 TB | ≈ 16 TB before overhead | 3 |
| 7 | 12 TB | 84 TB | ≈ 48 TB before overhead | 3 |
| 8 | 16 TB | 128 TB | ≈ 80 TB before overhead | 3 |
| 10 | 20 TB | 200 TB | ≈ 140 TB before overhead | 3 |
Examples use the simplified (N − 3) × drive-size model and are not exact formatted-capacity calculations.
Why Choose RAIDZ3?
RAIDZ2 vs RAIDZ3
| Characteristic | RAIDZ2 | RAIDZ3 |
|---|---|---|
| Parity | Double parity | Triple parity |
| Device Failures Tolerated | Up to 2 per VDEV | Up to 3 per VDEV |
| Parity Capacity | ≈ 2 drives in simplified model | ≈ 3 drives in simplified model |
| Capacity Efficiency | Higher | Lower |
| Fault Tolerance | High | Higher |
RAIDZ3 and Resilvering
When a failed RAIDZ3 device is replaced, ZFS restores the required information onto the replacement through resilvering. RAIDZ3's additional parity can be particularly valuable during this recovery period because the VDEV retains more device-failure tolerance after the initial failure than RAIDZ1 or RAIDZ2.
Resilver duration varies with allocated data, device performance, system activity, storage topology and hardware. RAIDZ3 reduces device-failure risk during recovery, but it does not eliminate the need to monitor and maintain the storage system.
RAIDZ3 Performance Considerations
RAIDZ3 should primarily be selected for its fault-tolerance characteristics. Actual performance depends on VDEV width, drive technology, workload, record sizes, memory and the number of data VDEVs in the pool.
Choosing Drives for RAIDZ3
RAIDZ3 protects against multiple device failures, but drive selection still matters. Capacity, interface, performance, endurance, workload rating and replacement availability should be considered when designing the pool.
Even RAIDZ3 Is Not a Backup
Triple parity provides substantial storage redundancy, but it does not protect against every form of data loss. Accidental deletion, administrative mistakes, malware, application-level corruption, catastrophic hardware damage and site-level events can still affect the data.
Critical data should still be protected with an independent backup strategy appropriate for the required recovery objectives.
RAIDZ3 FAQ
What is RAIDZ3?
RAIDZ3 is the triple-parity RAIDZ VDEV configuration in ZFS.
How many drives can RAIDZ3 lose?
A RAIDZ3 VDEV can tolerate up to three unavailable member devices.
How much capacity does RAIDZ3 use for parity?
In a simplified equal-drive model, approximately three drives' worth of raw capacity is devoted to parity.
Is RAIDZ3 safer than RAIDZ2?
RAIDZ3 provides an additional parity level, allowing three device failures instead of two within the VDEV.
Does RAIDZ3 have a dedicated parity disk?
No. RAIDZ distributes data and parity across the member devices rather than assigning permanent physical parity disks.
When should RAIDZ3 be considered?
RAIDZ3 can be attractive for large-capacity storage, important data and environments where additional protection during device replacement is valuable.
Can RAIDZ3 use SSDs?
Yes. RAIDZ3 can use supported SSDs or other block storage devices depending on the platform and ZFS implementation.
Is RAIDZ3 a backup?
No. Triple parity provides storage redundancy but does not replace an independent backup strategy.