2 × 8 TB in RAID 1: the numbers
| Usable space | 8 TB (7.3 TB in Windows) |
|---|---|
| Raw space | 16 TB |
| Used for protection | 8 TB |
| Efficiency | 50% |
| Drives that can fail | Any 1 |
| Rebuild at 150 MB/s | 14 h |
| Rebuild at 50 MB/s (busy) | 44 h |
| Data read during rebuild | 8 TB |
| Read error risk, desktop drives | 47% |
| Read error risk, NAS drives | 6.2% |
Rebuild
Replacing a failed drive means writing a full 8 TB drive. At 150 MB/s with no other load that takes about 14 h; on a busy array running at 50 MB/s, about 44 h. Only the mirror partner is read, so the other drives are not stressed.
Read errors
Reading 8 TB, the chance of at least one unreadable sector is 47% with desktop drives (1 per 1014 bits) and 6.2% 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 8 TB drives in RAID 5 would give 16 TB, and four in RAID 6 16 TB with two-drive protection.