12 × 2 TB in RAID 6: the numbers
| Usable space | 20 TB (18.2 TB in Windows) |
|---|---|
| Raw space | 24 TB |
| Used for protection | 4 TB |
| Efficiency | 83% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 3 h 42 min |
| Rebuild at 50 MB/s (busy) | 11 h |
| Data read during rebuild | 22 TB |
| Read error risk, desktop drives | 83% (corrected) |
| Read error risk, NAS drives | 16% (corrected) |
Rebuild
Replacing a failed drive means writing a full 2 TB drive. At 150 MB/s with no other load that takes about 3 h 42 min; on a busy array running at 50 MB/s, about 11 h. The rebuild reads every surviving drive: 22 TB in total.
Read errors
Reading 22 TB, the chance of at least one unreadable sector is 83% with desktop drives (1 per 1014 bits) and 16% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
Up to about 12 drives in one RAID 6 is common. Beyond that, two RAID 6 groups (RAID 60) keep rebuilds shorter. For 2 TB drives this is the setup most administrators would choose: the rebuild takes 3 h 42 min, and the array stays protected the whole time.