6 × 6 TB in RAID 6: the numbers
| Usable space | 24 TB (21.8 TB in Windows) |
|---|---|
| Raw space | 36 TB |
| Used for protection | 12 TB |
| Efficiency | 67% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 11 h |
| Rebuild at 50 MB/s (busy) | 33 h |
| Data read during rebuild | 30 TB |
| Read error risk, desktop drives | 91% (corrected) |
| Read error risk, NAS drives | 21% (corrected) |
Rebuild
Replacing a failed drive means writing a full 6 TB drive. At 150 MB/s with no other load that takes about 11 h; on a busy array running at 50 MB/s, about 33 h. The rebuild reads every surviving drive: 30 TB in total.
Read errors
Reading 30 TB, the chance of at least one unreadable sector is 91% with desktop drives (1 per 1014 bits) and 21% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
With 6 TB drives, RAID 5 would give 30 TB with a rebuild of 11 h. RAID 6 is still the safer choice for data you cannot replace.