4 × 20 TB in RAID 6: the numbers
| Usable space | 40 TB (36.4 TB in Windows) |
|---|---|
| Raw space | 80 TB |
| Used for protection | 40 TB |
| Efficiency | 50% |
| Drives that can fail | Any 2 |
| Rebuild at 150 MB/s | 37 h |
| Rebuild at 50 MB/s (busy) | 4.6 days |
| Data read during rebuild | 60 TB |
| Read error risk, desktop drives | 99% (corrected) |
| Read error risk, NAS drives | 38% (corrected) |
Rebuild
Replacing a failed drive means writing a full 20 TB drive. At 150 MB/s with no other load that takes about 37 h; on a busy array running at 50 MB/s, about 4.6 days. The rebuild reads every surviving drive: 60 TB in total.
Read errors
Reading 60 TB, the chance of at least one unreadable sector is 99% with desktop drives (1 per 1014 bits) and 38% 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 40 TB. RAID 6 survives any two failures; RAID 10 writes and rebuilds faster. For 20 TB drives this is the setup most administrators would choose: the rebuild takes 37 h, and the array stays protected the whole time.