Databases are write-heavy and latency-sensitive, so the best RAID minimises the write penalty and keeps latency predictable — which almost always means RAID 10 on flash. Size it in the RAID 10 calculator.
View the data behind this chart
| RAID 10 | RAID 5/6 | ZFS mirror | |
|---|---|---|---|
| Write penalty | ×2 | ×4 / ×6 | CoW |
| Write IOPS | Best | Throttled | Scales w/ vdevs |
| Latency | Predictable | Variable | Good |
| Verdict | Default | Read-heavy only | ZFS estates |
Why RAID 10 wins for databases
Transactional databases (SQL Server, Oracle, PostgreSQL, MySQL) issue lots of small random writes — commits, log flushes, index updates — and care deeply about latency. Parity RAID's write penalty (×4 RAID 5, ×6 RAID 6) directly throttles that, while RAID 10's ×2 penalty roughly doubles write IOPS and keeps latency predictable.
RAID 10 also rebuilds fast (mirror copy, no parity recompute), so performance stays high during a failure — important for an always-on database. The trade-off is 50% capacity, which databases usually justify.
Flash, and separating workloads
Use SSD/NVMe — databases are IOPS- and latency-bound, and flash removes the seek bottleneck. Classic practice separates data files, transaction logs and tempdb onto different volumes so sequential log writes don't contend with random data I/O; on a modern all-flash array this matters less but is still good hygiene for the busiest systems.
On ZFS, use mirror vdevs (not RAIDZ) for database datasets — a RAIDZ vdev only gives one drive's random IOPS. Tune recordsize/volblocksize to the database's block size (see ZFS tuning).
Sizing and protection
Size by peak write IOPS plus headroom, and confirm the delivered write IOPS in the calculator — compare RAID 10 against RAID 6 to see the penalty difference on your drives. For tier-0 databases, all-NVMe arrays (NetApp, Pure, HPE Alletra) are the enterprise route — our finder sizes them.
And databases above all need backups and point-in-time recovery — RAID protects the disk, not the data. See RAID is not a backup.