OpenMediaVault RAID Capacity Calculator
Work out raw versus usable capacity, parity overhead, the smaller TiB figure OpenMediaVault will actually report, and roughly how long a failed drive takes to rebuild.
A rebuild runs no faster than the drives it reads and writes, so the sustained sequential rate on your drives' datasheets is the ceiling, and a rebuild competing with normal use lands under it. SMR drives, a degraded array still serving clients, or a slow controller can drop the figure by an order of magnitude, which is why it is yours to set rather than ours to assume.
This is the window during which a second failure costs you the array on single-parity layouts. If it runs past a day, that is the argument for two-drive parity or mirrored pairs rather than for a bigger single-parity array.
Reading the numbers
Why the TiB figure is smaller. Drives are sold in decimal terabytes, where 1 TB is 1,000,000,000,000 bytes. Operating systems and the OpenMediaVault interface report binary tebibytes, where 1 TiB is 1,099,511,627,776 bytes. Nothing is missing and no capacity has been lost; the same bytes are being counted in different units, which is roughly a 9% difference. This is the single most common reason a new array looks smaller than expected.
Parity is not a backup. Parity covers a drive failing. It does not cover deletion, ransomware, a filesystem going bad, or the machine being stolen.
The rebuild estimate is straight arithmetic: one drive's worth of data divided by the rate you entered. Treat it as a planning figure, not a measurement.
This model assumes multiple data drives. For a Pi with one USB data disk, plan that disk and its backup separately; the OpenMediaVault on a Raspberry Pi guide covers the installation and storage checklist.
Related reading
- OpenMediaVault filesystems and shared folders — how the disk, filesystem, share and service layers stack up, and which array layout fails in which way.
- OpenMediaVault hardware requirements — what to buy, why CMR drives matter for rebuilds, and three reference builds.
- OpenMediaVault vs TrueNAS — plugins, storage choices and the Synology appliance alternative.
- Slow SMB transfers on OpenMediaVault — isolating the wire, the disks and the protocol when the array is slower than the plan.