7 × 24 TB in RAID 5: the numbers
| Usable space | 144 TB (131 TB in Windows) |
|---|---|
| Raw space | 168 TB |
| Used for protection | 24 TB |
| Efficiency | 86% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 44 h |
| Rebuild at 50 MB/s (busy) | 5.6 days |
| Data read during rebuild | 144 TB |
| Read error risk, desktop drives | over 99% |
| Read error risk, NAS drives | 68% |
Rebuild
Replacing a failed drive means writing a full 24 TB drive. At 150 MB/s with no other load that takes about 44 h; on a busy array running at 50 MB/s, about 5.6 days. The rebuild reads every surviving drive: 144 TB in total.
Read errors
Reading 144 TB, the chance of at least one unreadable sector is over 99% with desktop drives (1 per 1014 bits) and 68% 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 24 TB drives we would pick RAID 6. 8 drives in RAID 6 give the same 144 TB and survive two failures, or a read error during a rebuild. Keeping 7 drives, RAID 6 gives 120 TB.