2 × 2 TB in RAID 1: the numbers
| Usable space | 2 TB (1.8 TB in Windows) |
|---|---|
| Raw space | 4 TB |
| Used for protection | 2 TB |
| Efficiency | 50% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 3 h 42 min |
| Rebuild at 50 MB/s (busy) | 11 h |
| Data read during rebuild | 2 TB |
| Read error risk, desktop drives | 15% |
| Read error risk, NAS drives | 1.6% |
Rebuild
Replacing a failed drive means writing a full 2 TB drive. At 150 MB/s with no other load that takes about 3 h 42 min; on a busy array running at 50 MB/s, about 11 h. Only the mirror partner is read, so the other drives are not stressed.
Read errors
Reading 2 TB, the chance of at least one unreadable sector is 15% with desktop drives (1 per 1014 bits) and 1.6% 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 2 TB drives in RAID 5 would give 4 TB, and four in RAID 6 4 TB with two-drive protection.