Unraid capacity calculator
Unraid puts parity on the biggest drive and every other drive counts in full, which is why it's so forgiving of a drawer full of odd sizes. SnapRAID counts space the same way, and the numbers here hold for a DAS on a PC too.
Unraid, 1 parity · 4 drives · 44 TB raw
28TB
usable
≈ 25.5 TiB, which Windows calls “TB”
Survives 1 drive failing
- Usable
- 28 TB
- Protection
- 16 TB
- Unused
- 0 TB
- Parity lives on the largest drive (16 TB), and no data drive can be bigger than a parity drive.
- Lose more drives than you've got parity for and only those drives' files go. Everything else stays readable.
Same drives, every layout
| Layout | Usable | Survives | Of raw |
|---|---|---|---|
| 28 TB | 1 | 64% | |
| 16 TB | 2 | 36% | |
| 44 TB | 0 | 100% | |
| 8 TB | 3 | 18% | |
| 24 TB | 1 | 55% | |
| 16 TB | 2 | 36% | |
| 16 TB | 1–2 | 36% | |
| 20 TB | 1–2 | 45% | |
| 24 TB | 1 | 55% | |
| 16 TB | 2 | 36% | |
| – | needs 5+ | ||
| 28 TB | 1 | 64% | |
| 16 TB | 2 | 36% | |
| 28 TB | 1 | 64% | |
| 16 TB | 2 | 36% | |
| 44 TB | 0 | 100% |
“Survives” counts the drives that can fail at once before the pool goes. Where it says 1–2, a second failure is only survivable if it lands in a different mirror pair.
Unraid against ZFS on mixed drives
| Drives (TB) | 1 parity | 2 parity | RAIDZ1 |
|---|---|---|---|
| 8 + 8 + 12 + 16 | 28 TB | 16 TB | 24 TB |
| 12 + 16 + 16 + 20 + 20 | 64 TB | 44 TB | 48 TB |
| 4 + 8 + 8 + 12 + 16 + 16 | 48 TB | 32 TB | 20 TB |
No data drive can be bigger than parity. Buy your biggest drive first and make it the parity disk, then fill the rest with whatever's cheapest per TB. For the hardware, have a look at DAS enclosures and hot-swap cages.