4 × 8 TB in RAID 5: the numbers
| Usable space | 24 TB (21.8 TB in Windows) |
|---|---|
| Raw space | 32 TB |
| Used for protection | 8 TB |
| Efficiency | 75% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 14 h |
| Rebuild at 50 MB/s (busy) | 44 h |
| 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 8 TB drive. At 150 MB/s with no other load that takes about 14 h; on a busy array running at 50 MB/s, about 44 h. 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 8 TB drives we would pick RAID 6. 5 drives in RAID 6 give the same 24 TB and survive two failures, or a read error during a rebuild. Keeping 4 drives, RAID 6 gives 16 TB. RAID 10 with the same drives gives 16 TB, faster writes and shorter rebuilds.