I realised that one of my 3Tb drives was very, very old. It wasn’t failing yet, but it would probably be best to proactively replaced it. The other drive in the mirror is a 4Tb drive, so I took the opportunity to buy another 4Tb drive and add it to the ZFS mirror… This was so simple it’s scary. Firstly I removed the old drive from the mirror.
#zpool detach <Pool Name> <Device to Remove> i.e #zpool detach SATAPool sddThen I shutdown the machine removed the old drive and put the new one in. When the box rebooted I added the new disk to the mirror.
#zpool attach <Pool Name> <Existing Disk><New Disk> i.e #zpool attach SATAPool sdc sddI let it resilver… but it didn’t auto expand. The main reason for this was I didn’t have autoexpand turned on.
#zpool set autoexpand=on <Pool Name>This didn’t fix the issue, but it did show the space was available. The next thing I did was to use the online -e on every disk in the mirror.
#zpool online -e <Pool Name> <Disk Name> i.e. #zpool online -e SATAPool sddAnd then I had an extra Tb to play with!