7 × 18 TB in RAID 6: the numbers
| Usable space | 90 TB (81.9 TB in Windows) |
|---|---|
| Raw space | 126 TB |
| Used for protection | 36 TB |
| Efficiency | 71% |
| 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 | 108 TB |
| Read error risk, desktop drives | over 99% (corrected) |
| Read error risk, NAS drives | 58% (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: 108 TB in total.
Read errors
Reading 108 TB, the chance of at least one unreadable sector is over 99% with desktop drives (1 per 1014 bits) and 58% with NAS drives (1 per 1015). The second parity corrects it, so the rebuild finishes without losing data.
Our take
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.