1 · Choose a RAID level
Block striping with distributed parity. Survives one failure.
2 · Configure drives
3 · Drive class
3.5" nearline SAS/SATA capacity HDD — indicative figures.
Advanced — read/write mix, URE rate
Calculated for planning. We don't publish prices — a 24-year UK reseller, Servnet confirms the exact drives, array and pricing on quote. IOPS, throughput & rebuild are indicative estimates.
What RAID 5 is
RAID 5 stripes data with one distributed parity block per stripe, so usable capacity is (n−1) × drive size and the array survives a single drive failure. It is one of the most space-efficient redundant levels — five drives give 80% efficiency.
The catch is the rebuild. After a drive fails, RAID 5 must read every surviving drive in full to reconstruct the replacement, with no redundancy left. On large nearline drives, the chance of hitting an unrecoverable read error (URE) during that read becomes significant — which is why RAID 6 has largely replaced RAID 5 for big arrays. Use the calculator to see your own URE-on-rebuild figure.
Five 8 TB nearline drives give 32 TB usable at 80% efficiency. But a rebuild must read 32 TB across the survivors with no safety net — at a consumer 10¹⁴ URE rate that carries a real chance of a failed rebuild, which is the case for RAID 6 on drives this size.
Advantages
- High capacity efficiency — (n−1)/n
- Survives one drive failure
- Good read performance
- Widely supported by every controller
Trade-offs
- ×4 write penalty (read-modify-write)
- No redundancy during rebuild — a URE means data loss
- Long, risky rebuilds on large drives
- Cannot survive a second failure
Best for
- Smaller arrays of modest-capacity drives
- Read-heavy workloads
- Where capacity efficiency matters and a backup exists
Consider another level when
- Large nearline (8 TB+) HDD arrays — use RAID 6
- Write-heavy databases
- Mission-critical data without a backup
RAID 5 — common questions
How is RAID 5 usable capacity calculated?
Usable capacity is (number of drives − 1) × drive size, because one drive’s worth of capacity is consumed by distributed parity. Five 8 TB drives give (5−1) × 8 = 32 TB usable, an 80% efficiency.
Why do people say RAID 5 is dead?
On large drives, a single-drive rebuild reads all surviving drives with no redundancy left. If an unrecoverable read error (URE) occurs during that read, the rebuild fails and data is lost. The bigger the drives, the higher that probability — the calculator shows it for your config. RAID 6 (dual parity) survives a URE during a single-drive rebuild.
What is the RAID 5 write penalty?
RAID 5 has a ×4 write penalty: each random write requires reading the old data and parity, then writing the new data and parity — four back-end I/Os per host write. This is why RAID 10 is preferred for write-heavy databases.