What each option changes
The calculator needs five details about the block and the RAIDZ vdev that will store it.
| Option | What it means |
|---|---|
| RAIDZ level | RAIDZ1, RAIDZ2, or RAIDZ3. The number tells you how many parity sectors each stripe row needs. |
| Vdev width | The total number of drives in this RAIDZ vdev. |
| Workload | Choose a filesystem dataset or a zvol. This decides whether the block setting is called recordsize or volblocksize. |
| Block size | The logical size of the one block being tested. |
ashift | The physical sector size ZFS uses for allocation. For example, ashift=12 means 4 KiB sectors. |
recordsize and volblocksize describe different blocks
recordsize is the largest block ZFS normally uses for file data in a dataset. A small file or the end of a file can use a smaller block. volblocksize is the block size used by a zvol. The calculator changes the name and valid range when you switch between these workloads.
Why an 8 KiB block can occupy 24 KiB
Consider a six-drive RAIDZ2 vdev with ashift=12. Each sector is 4 KiB, so an 8 KiB logical block starts as two data sectors. RAIDZ2 adds two parity sectors, then ZFS adds two padding sectors to complete the allocation.
| Part | Sectors | Space |
|---|---|---|
| Logical data | 2 | 8 KiB |
| Sector rounding | 0 | 0 B |
| Parity | 2 | 8 KiB |
| Padding | 2 | 8 KiB |
| Physical allocation | 6 | 24 KiB |
Space efficiency is not a performance recommendation
A block size can use space efficiently in this calculation and still be a poor fit for your workload. Block size also affects reads, writes, compression opportunities, and how an application accesses data. Use the result to understand allocation, not as a universal setting for every system.
What this calculator leaves out
This is a theoretical calculation for one uncompressed block on one RAIDZ vdev. It does not calculate:
- Compression, deduplication, or extra copies.
- Metadata, snapshots, reservations, slop space, or fragmentation.
- Special vdevs, dRAID, gang blocks, or expansion history.
- Whole-pool capacity, real free space, or storage performance.
Your calculation stays in your browser
Typing values and calculating do not send your configuration to a server. Copy and share actions use the result only when you choose them. A shared link includes the configuration in its URL.
Need whole-pool capacity?
Use the ZFS pool capacity calculator to combine top-level vdevs and estimate writable space, redundancy, slop, and planning headroom.