3 × 12 TB in RAID 5: the numbers
| Usable space | 24 TB (21.8 TB in Windows) |
|---|---|
| Raw space | 36 TB |
| Used for protection | 12 TB |
| Efficiency | 67% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 22 h |
| Rebuild at 50 MB/s (busy) | 2.8 days |
| Data read during rebuild | 24 TB |
| Read error risk, desktop drives | 85% |
| Read error risk, NAS drives | 17% |
Rebuild
Replacing a failed drive means writing a full 12 TB drive. At 150 MB/s with no other load that takes about 22 h; on a busy array running at 50 MB/s, about 2.8 days. The rebuild reads every surviving drive: 24 TB in total.
Read errors
Reading 24 TB, the chance of at least one unreadable sector is 85% with desktop drives (1 per 1014 bits) and 17% with NAS drives (1 per 1015). Nothing is left to correct it, so the affected file, or on some controllers the whole rebuild, fails. These are worst-case datasheet rates; real drives usually do better.
Our take
With 12 TB drives we would pick RAID 6. 4 drives in RAID 6 give the same 24 TB and survive two failures, or a read error during a rebuild.