Before you start
- The principles of RAID and NAS are explained in Understanding storage, RAID and NAS
In plain words
RAID spreads your data over several disks so it survives a failure. But each protection disk is a disk that does not store your files. The calculator shows what you really keep, and how many disks can fail before you lose everything. Everything is calculated in your browser.
Rules of thumb
| Situation | Usual choice |
|---|---|
| 2 disks | Mirror (RAID 1 or ZFS mirror) |
| 4 disks, performance first (virtual machines) | RAID 10 or two ZFS mirrors |
| 4 to 8 disks of 4 TB and more, file storage | RAID 6 or RAIDZ2 |
| More than 10 disks | Several RAIDZ2 vdevs |
| Disks of different sizes, added over time | Unraid |
| Disposable data, maximum speed | RAID 0, only with a backup |
TB or TiB?
Manufacturers count in decimal TB (1 TB = 1,000 billion bytes), operating systems in binary TiB (1 TiB = 2⁴⁰ bytes, or 1.0995 TB). So an 8 TB disk shows up as 7.28 "TB" in Windows or on a NAS: nothing has disappeared, only the unit has changed. The calculator shows both.
The assumptions behind the calculation
- All disks have the same capacity (except for Unraid, which accepts different sizes).
- For ZFS, the pool reserve is deducted: 1/32 of the space (about 3.1%), capped at 128 GiB since OpenZFS 2.0.6, which makes it negligible on large pools; the real space in RAIDZ can be slightly lower depending on block size. ZFS compression, on by default, often lets you store more than the capacity shown.
- The rebuild time is a minimum, calculated at 150 MB/s for a hard disk: on a NAS in use, it often takes two to three times longer.
RAID is not a backup
A deletion, ransomware or a mistake is replicated instantly to every disk. Always keep a separate copy, including one off-site: see The 3-2-1 rule.
To choose the NAS operating system, see TrueNAS, OpenMediaVault or Unraid.