vdevs

A vdev, short for virtual device, is a fundamental unit of a ZFS storage pool (zpool). Each vdev is a group of one or more physical disks that ZFS uses to store data with a chosen level of redundancy. The capacity of a pool is the sum of the usable capacities of its vdevs, while redundancy and performance are determined by the vdev layouts.

Common vdev layouts include a single-disk vdev (no redundancy), a mirrored vdev (two or more disks with copies of data), and RAID-Z vdevs (RAID-Z1, RAID-Z2, RAID-Z3) that provide parity-based redundancy. A pool can contain multiple vdevs of different types, and data is striped across vdevs, which can help with I/O parallelism.

Redundancy is implemented at the vdev level. If a disk fails within a vdev, the pool may remain online if the vdev’s redundancy is preserved, such as in a mirror, or the pool may become degraded or unavailable if redundancy is exceeded, as can happen with RAID-Z configurations when multiple failures occur. If a vdev becomes unavailable, the pool is typically degraded and can fail if enough redundancy is lost to prevent reconstruction.

Administratively, vdevs can be expanded by adding new vdevs to the pool to increase capacity. Existing vdevs can be upgraded or replaced by inserting new disks and performing resilvering. Detaching a disk from a mirror reduces redundancy but allows replacement without data loss; removing a vdev typically requires moving data off the pool or destroying it.

Hot spares can be configured to automatically replace failed disks, improving availability. Vdev choices influence performance, capacity, and fault tolerance, making them central to ZFS storage design.