ZFS Capacity Calculator

How the ZFS capacity calculator works

A ZFS pool consists of one or more vdevs (virtual devices). Usable capacity depends on how those vdevs are built. The calculator derives the realistic net capacity from the number of vdevs, their disks and the redundancy type – including the crucial conversion from marketing terabytes to the tebibytes ZFS actually uses.

• TB vs. TiB: Disks are sold in decimal terabytes (base 1000), while ZFS counts in binary tebibytes (base 1024). A “12 TB” drive only yields about 10.9 TiB.
• Mirror: Mirrors the data. A 2-way mirror uses 50% of the raw capacity and survives one disk failure; a 3-way mirror survives two.
• RAIDZ1/2/3: Stripes data plus 1, 2 or 3 parity disks per vdev. Usable space is (disks − parity) × disk size per vdev.
• Stripe: Maximum capacity but no redundancy at all – losing a single disk destroys the pool.
• 80% fill level: ZFS slows down noticeably beyond roughly 80% usage (copy-on-write, fragmentation). Plan your usable capacity accordingly lower.

Important note on fault tolerance

The stated fault tolerance applies per vdev. If a third disk fails in a RAIDZ2 vdev before the first two are replaced, the entire pool is lost – even if other vdevs are intact. So adding vdevs increases capacity and performance, but not the fault tolerance of the individual vdev.

Related calculators

• Ceph Capacity Calculator
• RAID Calculator