4 × 2 TB in RAID 6: the numbers
| Usable space | 4 TB (3.6 TB in Windows) |
|---|---|
| Raw space | 8 TB |
| Used for protection | 4 TB |
| Efficiency | 50% |
| 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 | 6 TB |
| Read error risk, desktop drives | 38% (corrected) |
| Read error risk, NAS drives | 4.7% (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: 6 TB in total.
Read errors
Reading 6 TB, the chance of at least one unreadable sector is 38% with desktop drives (1 per 1014 bits) and 4.7% 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 4 TB. RAID 6 survives any two failures; RAID 10 writes and rebuilds faster. With 2 TB drives, RAID 5 would give 6 TB with a rebuild of 3 h 42 min. RAID 6 is still the safer choice for data you cannot replace.