7 × 8 TB in RAID 6: the numbers
| Usable space | 40 TB (36.4 TB in Windows) |
|---|---|
| Raw space | 56 TB |
| Used for protection | 16 TB |
| Efficiency | 71% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 14 h |
| Rebuild at 50 MB/s (busy) | 44 h |
| Data read during rebuild | 48 TB |
| Read error risk, desktop drives | 98% (corrected) |
| Read error risk, NAS drives | 32% (corrected) |
Rebuild
Replacing a failed drive means writing a full 8 TB drive. At 150 MB/s with no other load that takes about 14 h; on a busy array running at 50 MB/s, about 44 h. The rebuild reads every surviving drive: 48 TB in total.
Read errors
Reading 48 TB, the chance of at least one unreadable sector is 98% with desktop drives (1 per 1014 bits) and 32% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
For 8 TB drives this is the setup most administrators would choose: the rebuild takes 14 h, and the array stays protected the whole time.