Solaris ZFS Administration Guide

Dynamic Striping in a Storage Pool

ZFS dynamically stripes data across all top-level virtualdevices. The decision about where to place data is done at write time, so no fixed width stripes are created at allocation time.

When new virtual devices are added to a pool, ZFS gradually allocates data to the new device in order to maintain performance and space allocation policies. Each virtual device can also be a mirror or a RAID-Z device that contains other disk devices or files. This configuration allows for flexibility in controlling the fault characteristics of your pool. For example, you could create the following configurations out of 4 disks:

While ZFS supports combining different types of virtual devices within the same pool, this practice is not recommended. For example, you can create a pool with a two-way mirror and a three-way RAID-Z configuration. However, your fault tolerance is as good as your worst virtual device, RAID-Z in this case. The recommended practice is to use top-level virtual devices of the same type with the same redundancy level in each device.