7 × 10 TB in RAID 5: the numbers
| Usable space | 60 TB (54.6 TB in Windows) |
|---|---|
| Raw space | 70 TB |
| Used for protection | 10 TB |
| Efficiency | 86% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 18 h |
| Rebuild at 50 MB/s (busy) | 2.3 days |
| Data read during rebuild | 60 TB |
| Read error risk, desktop drives | 99% |
| Read error risk, NAS drives | 38% |
Rebuild
Replacing a failed drive means writing a full 10 TB drive. At 150 MB/s with no other load that takes about 18 h; on a busy array running at 50 MB/s, about 2.3 days. The rebuild reads every surviving drive: 60 TB in total.
Read errors
Reading 60 TB, the chance of at least one unreadable sector is 99% with desktop drives (1 per 1014 bits) and 38% 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 10 TB drives we would pick RAID 6. 8 drives in RAID 6 give the same 60 TB and survive two failures, or a read error during a rebuild. Keeping 7 drives, RAID 6 gives 50 TB.