volblocksize

Volblocksize is a ZFS property that controls the logical block size of a zvol, a ZFS volume exposed as a block device. The value determines how data is organized on the zvol’s backing store and how the host I/O is issued to the device. The volblocksize is set when the zvol is created and is expressed in bytes. It is usually a power-of-two value within a range that commonly spans from 512 bytes up to 1 MiB, chosen to match the workload and the storage subsystem.

The choice of volblocksize affects performance. Smaller values improve random I/O efficiency and reduce wasted space for small writes, but increase metadata overhead and CPU work. Larger values favor sequential, large-block I/O and can improve throughput for workloads that perform large writes, such as virtual machine disks, at the cost of potential wasted space for small writes. The right value depends on the workload and hardware characteristics.

To achieve good results, volblocksize should be aligned with the host’s I/O size and the storage system’s characteristics, including the pool’s alignment and underlying sector size (ashift). For virtual machines, common starting points are 4K or 8K; for large sequential I/O, 16K or 32K may be considered. However, the optimal choice varies by environment.

Changing volblocksize on an existing zvol is generally not feasible and typically requires recreating the zvol to apply a new block size. The volblocksize is specified at creation with commands such as zfs create -V size -o volblocksize=value pool/vol. After creation, always consult current ZFS documentation for any exceptions.

Volblocksize interacts with other features like compression and deduplication, since I/O size and alignment influence data layout, compression efficiency, and metadata management. Administrators select volblocksize to optimize the intended workload on the zvol.