STORAGE / CAPACITY PLANNING
RAID / ZFS Calculator
Compare redundancy layouts before committing your disks. See the assumptions behind every estimate.
NOMINAL CAPACITY AFTER REDUNDANCY
- Raw disk capacity
- 48 TB / 43.66 TiB
- With planning reserve
- 25.6 TB / 23.28 TiB
- Selected reserve
- 20%
Nominally 2 disk failures per vdev. Exceeding a vdev's redundancy can lose the whole pool.
Estimates exclude filesystem metadata, RAIDZ padding, ZFS slop space, snapshots, spares, compression, and workload-specific effects. Redundancy does not replace a backup.
What this calculator estimates
The model starts with equal-size active disks. For mixed disks, use the smallest member capacity as a simplified planning input; actual controller and pool behaviour can differ. Exclude hot spares and dedicated cache, log, or special devices from the data-disk count.
The result shows nominal space after the selected redundancy scheme, then subtracts a user-selected reserve. That reserve is a planning choice, not a measured filesystem overhead or a universal utilisation recommendation.
Layout formulas
| Layout | Nominal capacity |
|---|---|
| RAID 0 | N × disk capacity |
| RAID 1, one N-way mirror | One disk capacity |
| RAID 5 / RAID 6 | (N − 1) / (N − 2) × disk capacity |
| RAID 10 / two-way ZFS mirrors | N ÷ 2 × disk capacity |
| Equal-width RAIDZ vdevs | V × (D − P) × disk capacity |
For RAIDZ, V is the vdev count, D the disks per vdev, and P the parity level. This model supports at least two data disks per RAIDZ vdev; it does not enumerate every technically permitted ZFS topology. RAID 10 is modelled as equal two-disk mirror pairs.
Example: six 8 TB disks in RAIDZ2
One six-disk RAIDZ2 vdev has 48 TB raw capacity and approximately 32 TB nominal capacity after two parity-equivalent disks. A selected 20% reserve reduces the planning figure to 25.6 TB, before the omitted effects above.
Why are TB and TiB different?
One TB is 1,000,000,000,000 bytes; one TiB is 1,099,511,627,776 bytes. Different numbers can describe the same byte count. This tool reports both units.
Can I lose any two disks with mirrored pairs?
No. Which pair loses members matters. Losing both disks in one two-way mirror is different from losing one member in each of two mirrors.
Does parity guarantee recovery?
No. The displayed tolerance describes the nominal topology, not a guarantee against unreadable remaining data, operator error, or correlated failures.
Plan recovery too
Use the Proxmox backup and restore checklist alongside the capacity plan. Check the real pool's reported allocation and workload before migrating important data.
Estimate a transfer window
Capacity and recovery time answer different questions. Use the download and upload time calculator to estimate a backup transfer or calculate the connection speed needed for a target window.