Start with the drives and the failure requirement
Enter the drives that will belong to the same array or pool. Keep the unit printed for each capacity: TB and TiB represent different quantities of bytes.
For striped mirrors, row order matters because the model pairs drives in that order. Pairing a smaller drive with a larger one leaves some of the larger member outside that mirror's usable capacity. The topology helps you check whether those are the pairs you actually intend to build.
Set the minimum protected-failure target before comparing the remaining layouts. The workload choice changes the ranking preference, not the byte calculation or the protection a layout provides.
How should I choose between eligible layouts?
Read capacity alongside the failure boundary. With four equal-size drives, a two-pair mirror layout and a dual-parity layout can both use two drive-equivalents for data. Their failure behavior is different:
| Layout | Why the failure placement matters |
|---|---|
| Two striped mirror pairs | One failed member in each pair can leave data accessible; losing both members of one pair can lose the pool |
| Dual parity | The model tolerates any two drive failures in that group |
A capacity tie therefore doesn't make the layouts interchangeable. A workload preference may rank one higher, but that is a stated selection rule, not a measured speed result or a reliability forecast.
"Best fit" means best fit among the candidates this tool evaluates for your inputs. It doesn't compare every storage technology, inspect your machine, or promise that the first row is safe for every workload.
What does the capacity breakdown include?
The model accounts for the entered raw capacity as usable space, redundancy, and mixed-size mismatch. The topology identifies which drives share a protection group. Those exact values describe the proposed layout before filesystem-specific deductions.
It does not inspect partitions, snapshots, compression, allocation history, rebuild behavior, or the expansion operations supported by an installed platform. A theoretical layout that fits the inventory might still be unavailable on your NAS or impossible to reach from your current pool without migration.
Verify the platform and recovery plan
Check supported layout names, minimum widths, expansion restrictions, and recovery procedures before creating storage. Use the Linux MD documentation or OpenZFS's RAIDZ and vdev references for the system you intend to run.
For a detailed ZFS estimate, continue with the ZFS capacity calculator. If the question is adding a disk to an existing RAIDZ vdev, use the separate RAIDZ expansion planner. Neither capacity nor parity proves that an independent backup will restore.
Calculation stays in your browser without saving inputs to local storage. Choosing Share puts the configuration in a URL that anyone with the link can read.