1 · Choose a RAID level
Block striping with dual distributed parity. Survives two failures.
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 6 is
RAID 6 adds a second, independent parity block per stripe, so usable capacity is (n−2) × drive size and the array survives any two simultaneous drive failures. Eight 16 TB drives give 96 TB usable at 75% efficiency.
That second parity is what makes RAID 6 the modern default for large nearline arrays: during a single-drive rebuild there is still one parity left, so an unrecoverable read error (URE) on a surviving drive is reconstructed rather than fatal. The cost is a ×6 write penalty, so RAID 6 favours capacity and read-heavy workloads over write-intensive databases.
Eight 16 TB drives in RAID 6 give 96 TB usable at 75% efficiency and tolerate any two drive losses. Crucially, during a single-drive rebuild the remaining parity still covers a URE — the failure mode that kills RAID 5 on drives this large.
Advantages
- Survives two simultaneous failures
- A URE during a single-drive rebuild is recoverable
- Strong capacity efficiency — (n−2)/n
- Safe default for large-capacity HDD pools
Trade-offs
- ×6 write penalty — heaviest of the common levels
- Two drives of capacity lost to parity
- Slower rebuilds than mirrors
- More controller overhead than RAID 5
Best for
- Large nearline / capacity HDD arrays
- Bulk file, backup-target and media storage
- Read-heavy workloads needing resilience
- Anywhere RAID 5 rebuild risk is unacceptable
Consider another level when
- Write-heavy transactional databases (use RAID 10)
- Latency-critical tier-0 workloads
- Very small arrays (the overhead is proportionally high)
RAID 6 — common questions
How is RAID 6 usable capacity calculated?
Usable capacity is (number of drives − 2) × drive size, because two drives’ worth of capacity holds dual parity. Eight 16 TB drives give (8−2) × 16 = 96 TB usable, a 75% efficiency.
Is RAID 6 safe for a URE during rebuild?
Yes — that is its main advantage over RAID 5. While rebuilding a single failed drive, RAID 6 still has a second parity, so an unrecoverable read error on a surviving drive is reconstructed and the rebuild continues. Data loss requires a concurrent second drive failure.
RAID 6 vs RAID 10 for databases?
RAID 6 wins on capacity and survives any two failures, but its ×6 write penalty hurts write-heavy databases. RAID 10 (×2 penalty) gives far better write performance and faster rebuilds, at 50% efficiency. Pick RAID 6 for capacity/read, RAID 10 for write-heavy/latency-sensitive.