4 × 18 TB in RAID 5: the numbers
| Usable space | 54 TB (49.1 TB in Windows) |
|---|---|
| Raw space | 72 TB |
| Used for protection | 18 TB |
| Efficiency | 75% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 33 h |
| Rebuild at 50 MB/s (busy) | 4.2 days |
| Data read during rebuild | 54 TB |
| Read error risk, desktop drives | 99% |
| Read error risk, NAS drives | 35% |
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: 54 TB in total.
Read errors
Reading 54 TB, the chance of at least one unreadable sector is 99% with desktop drives (1 per 1014 bits) and 35% 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 18 TB drives we would pick RAID 6. 5 drives in RAID 6 give the same 54 TB and survive two failures, or a read error during a rebuild. Keeping 4 drives, RAID 6 gives 36 TB. RAID 10 with the same drives gives 36 TB, faster writes and shorter rebuilds.