vdev

A vdev, short for virtual device, is the fundamental building block of a ZFS storage pool (zpool). In ZFS, a vdev is an abstraction that groups one or more physical storage devices (disks or partitions) and presents them to the pool as a single device for I/O operations. A pool is composed of one or more vdevs, and data and metadata are striped across the vdevs in the pool to provide capacity and performance.

Vdev configurations define redundancy and risk tolerance. Common types include:

- Single-disk vdev: no redundancy; data is stored on one device.

- Mirror vdev: two or more disks hold copies of each data block, providing fault tolerance.

- RAID-Z vdevs: RAID-Z1, RAID-Z2, or RAID-Z3 configurations with distributed parity, allowing one, two, or three disk failures per vdev, respectively.

The overall resilience of a zpool depends on its vdevs. If a vdev fails beyond its redundancy, the data stored on that vdev becomes unavailable, and pool health can degrade. With multiple healthy vdevs, the pool can continue operating in a degraded state; data loss occurs if enough devices fail across vdevs to exceed the pool’s redundancy.

Vdevs are defined at pool creation and typically can be expanded by adding new vdevs to the pool. Individual vdevs are typically not grown by simply adding disks to an existing vdev; instead, disks can be replaced for resiliency, or new vdevs can be added to increase capacity and redundancy.

In practice, vdev design strongly influences performance and fault tolerance, and it is a central consideration when configuring or expanding a ZFS storage system.