6 × 4 TB in RAID 6: the numbers
| Usable space | 16 TB (14.6 TB in Windows) |
|---|---|
| Raw space | 24 TB |
| Used for protection | 8 TB |
| Efficiency | 67% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 7 h 24 min |
| Rebuild at 50 MB/s (busy) | 22 h |
| Data read during rebuild | 20 TB |
| Read error risk, desktop drives | 80% (corrected) |
| Read error risk, NAS drives | 15% (corrected) |
Rebuild
Replacing a failed drive means writing a full 4 TB drive. At 150 MB/s with no other load that takes about 7 h 24 min; on a busy array running at 50 MB/s, about 22 h. The rebuild reads every surviving drive: 20 TB in total.
Read errors
Reading 20 TB, the chance of at least one unreadable sector is 80% with desktop drives (1 per 1014 bits) and 15% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
With 4 TB drives, RAID 5 would give 20 TB with a rebuild of 7 h 24 min. RAID 6 is still the safer choice for data you cannot replace.