Tools
RAID calculator
Pick the drives and the RAID level. The diagram shows how data and parity sit across the drives, which makes it easier to see why some layouts survive two failures and others do not.
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Result
Four stripes across the drives
How the RAID calculator works
Hot spares are taken out first. With n remaining drives of size S: RAID 0 = n × S, RAID 1 = S (every drive holds a full copy), RAID 5 and RAIDZ1 = (n - 1) × S, RAID 6 and RAIDZ2 = (n - 2) × S, RAIDZ3 = (n - 3) × S, RAID 10 = n ÷ 2 × S.
Drive makers use decimal terabytes (10^12 bytes). Windows and many storage tools report binary tebibytes (2^40 bytes), often labelled TB, which is about 9% less. That is why a new array looks smaller than expected.
Rebuild time = drive size ÷ rebuild speed, which assumes a full drive. ZFS resilvers only the space in use, so a part full pool rebuilds sooner. Real rebuilds run slower while the array serves users. ZFS pools also keep some space free for performance, and file system overhead is not deducted here.
Planning estimates. They do not replace a measurement on your own systems.
In practice
Worked example: six 8 TB drives in RAID 6 give 32 TB usable, which Windows shows as 29.1 TiB. Any two drives can fail. At 100 MB/s, rebuilding one drive takes about 22 hours.
RAID protects against a failed drive, not against deletion, ransomware or a failed controller. It is never a substitute for a backup.
| Level | Minimum drives | Drives that can fail | Usable capacity |
|---|---|---|---|
| RAID 0 | 2 | 0 | n × S |
| RAID 1 | 2 | n - 1 | 1 × S |
| RAID 5 | 3 | 1 | (n - 1) × S |
| RAID 6 | 4 | 2 | (n - 2) × S |
| RAID 10 | 4, even | 1 to n ÷ 2 | n ÷ 2 × S |
| RAIDZ1 | 3 | 1 | (n - 1) × S |
| RAIDZ2 | 4 | 2 | (n - 2) × S |
| RAIDZ3 | 5 | 3 | (n - 3) × S |
Questions about this tool
RAID 5 or RAID 6?
With drives of 4 TB and larger, RAID 6 or RAIDZ2. Rebuilds take many hours, and during that time RAID 5 has no protection left.
Why does a new array look smaller?
Drive makers count in decimal terabytes and Windows in binary units, which are about 9% smaller. File system overhead and reserved space take a little more.
Is RAID 10 better than RAID 6?
RAID 10 writes faster and rebuilds quicker because it only copies a mirror, but it gives up half the capacity and is only guaranteed to survive one failure. For large storage that is mostly read, RAID 6 is usually the better trade.
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