8 × 4 TB in RAID 5: the numbers
| Usable space | 28 TB (25.5 TB in Windows) |
|---|---|
| Raw space | 32 TB |
| Used for protection | 4 TB |
| Efficiency | 88% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 7 h 24 min |
| Rebuild at 50 MB/s (busy) | 22 h |
| Data read during rebuild | 28 TB |
| Read error risk, desktop drives | 89% |
| Read error risk, NAS drives | 20% |
Rebuild
Replacing a failed drive means writing a full 4 TB drive. At 150 MB/s with no other load that takes about 7 h 24 min; on a busy array running at 50 MB/s, about 22 h. The rebuild reads every surviving drive: 28 TB in total.
Read errors
Reading 28 TB, the chance of at least one unreadable sector is 89% with desktop drives (1 per 1014 bits) and 20% 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 drives sharing one parity, a second failure during the rebuild gets more likely. RAID 6 gives 24 TB with two-drive protection. RAID 10 with the same drives gives 16 TB, faster writes and shorter rebuilds.