💽 RAID / NAS Capacity

How much space you really get from your disks with RAID 0, 1, 5, 6, 10 or Synology SHR – and how many disks can fail before the data is gone.

Disks

Disk sizes [TB] separated by commas, e.g. 4, 4, 4, 8

Usable capacity

TB is what is written on the disk (10¹² bytes). Windows and many NAS systems show TiB (2⁴⁰ bytes), so the same space looks about 9 % smaller. File system and system partitions take a little more.

The RAID levels in short

RAID 0 – data split across all disks. Fast, all space usable, but one dead disk = everything lost.
RAID 1 – every disk holds the same data (mirror). Usable space = the smallest disk.
RAID 5 – one disk worth of parity, from 3 disks. Survives one failed disk. With today's large disks the rebuild takes long, and a second failure in that time is fatal.
RAID 6 – two disks worth of parity, from 4 disks. Survives two failed disks.
RAID 10 – mirrored pairs, striped. Fast; survives one failure per pair.
SHR-1 / SHR-2 – Synology Hybrid RAID. Like RAID 5 / 6, but uses disks of different sizes better: roughly the total minus the largest disk (SHR-1) or minus the two largest (SHR-2).

Classic RAID uses only as much of each disk as the smallest one has. And once more: RAID is not a backup – a deleted or encrypted file is deleted or encrypted on all disks at once. See Constructing a Backup.