When new virtual devices are added to a pool, ZFS gradually allocates data to the new device in order to maintain performance and disk 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 gives you flexibility in controlling the fault characteristics of your pool. For example, you could create the following configurations out of four disks:
Four disks using dynamic striping
One four-way RAID-Z configuration
Two two-way mirrors using dynamic striping
Although ZFS supports combining different types of virtual devices within the same pool, avoid this practice. 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. A best practice is to use top-level virtual devices of the same type with the same redundancy level in each device.