Danger of accidentally adding their ZIL or L2ARC to Zpool as a member disk
When using the [[FreeNAS]] web GUI to add a ZIL, or an L2ARC to an existing zpool there is a danger of adding this device as a member disk of a vdev.
For instance after selecting volume to extend. One can identically select the zil as a member disk rather than a log device or cache device in zfs extra.
The GUI should validate that a user hasn't selected a device for both a pool disk and an extra function (cache, log, or spare) and show an error if attempted. There's multiple possible implementations of this constraint.
We need something done to keep people from permanently breaking their pools while trying to perform a regular maintenance operation.