2 × 6 TB in RAID 1: the numbers
| Usable space | 6 TB (5.5 TB in Windows) |
|---|---|
| Raw space | 12 TB |
| Used for protection | 6 TB |
| Efficiency | 50% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 11 h |
| Rebuild at 50 MB/s (busy) | 33 h |
| Data read during rebuild | 6 TB |
| Read error risk, desktop drives | 38% |
| Read error risk, NAS drives | 4.7% |
Rebuild
Replacing a failed drive means writing a full 6 TB drive. At 150 MB/s with no other load that takes about 11 h; on a busy array running at 50 MB/s, about 33 h. Only the mirror partner is read, so the other drives are not stressed.
Read errors
Reading 6 TB, the chance of at least one unreadable sector is 38% with desktop drives (1 per 1014 bits) and 4.7% 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
A mirror is the simplest safe setup for two drives, and either drive can usually be read on its own. If you add drives later, three 6 TB drives in RAID 5 would give 12 TB, and four in RAID 6 12 TB with two-drive protection.