4 × 18 TB in RAID 6: the numbers
| Usable space | 36 TB (32.7 TB in Windows) |
|---|---|
| Raw space | 72 TB |
| Used for protection | 36 TB |
| Efficiency | 50% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 33 h |
| Rebuild at 50 MB/s (busy) | 4.2 days |
| Data read during rebuild | 54 TB |
| Read error risk, desktop drives | 99% (corrected) |
| Read error risk, NAS drives | 35% (corrected) |
Rebuild
Replacing a failed drive means writing a full 18 TB drive. At 150 MB/s with no other load that takes about 33 h; on a busy array running at 50 MB/s, about 4.2 days. The rebuild reads every surviving drive: 54 TB in total.
Read errors
Reading 54 TB, the chance of at least one unreadable sector is 99% with desktop drives (1 per 1014 bits) and 35% 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 36 TB. RAID 6 survives any two failures; RAID 10 writes and rebuilds faster. For 18 TB drives this is the setup most administrators would choose: the rebuild takes 33 h, and the array stays protected the whole time.