ZFS - Pools - Expand a ZFS Pool

zpool add - This adds a vdev to a pool. This generally results in more storage space.

zpool attach - This attaches a device to a vdev in the pool. This generally results in more redundancy.

Adding vdevs

Adding a RAID Z1 vdev to a pool

zpool add testpool raidz1 ada4 ada5 ada6

Adding a mirrored vdev to a pool:

zpool add testpool mirror ada4 ada5 ada6
zpool add otherpool ada4

NOTE: This will increase capacity.

A Disk can be attached.

zpool attach testpool ada1 ada4

NOTE: This will add ada4 to whichever vdev has ada1.

A disk can also be detached:

zpool detach testpool ada4