8 × 20 TB in RAID 6: the numbers
| Usable space | 120 TB (109.1 TB in Windows) |
|---|---|
| Raw space | 160 TB |
| Used for protection | 40 TB |
| Efficiency | 75% |
| 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 | 140 TB |
| Read error risk, desktop drives | over 99% (corrected) |
| Read error risk, NAS drives | 67% (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: 140 TB in total.
Read errors
Reading 140 TB, the chance of at least one unreadable sector is over 99% with desktop drives (1 per 1014 bits) and 67% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
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.