4 × 8 TB in RAID 6: the numbers
| Usable space | 16 TB (14.6 TB in Windows) |
|---|---|
| Raw space | 32 TB |
| Used for protection | 16 TB |
| Efficiency | 50% |
| 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 | 24 TB |
| Read error risk, desktop drives | 85% (corrected) |
| Read error risk, NAS drives | 17% (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: 24 TB in total.
Read errors
Reading 24 TB, the chance of at least one unreadable sector is 85% with desktop drives (1 per 1014 bits) and 17% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
With four drives, RAID 6 and RAID 10 both give 16 TB. RAID 6 survives any two failures; RAID 10 writes and rebuilds faster. 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.