Here is the procedure for replacing a write log ZIL SSDs drive that has failed.
1. In the web user interface, go into 'Storage Pools', and right click the storage pool you wish to remove the SSDs from, and then click 'Remove Cache Device'.
2. Tick the write log SSD devices you wish to remove, and then click 'OK'.
3. Remove the write log SSD drive from the enclosure, and then insert the new one.
4. Go into 'Physical Disks' and click 'Scan'
5. Go into 'Storage Pools', right click the storage pool you wish to add the SSDs to, and click 'Add Cache Device'.
6. Tick the write log SSD devices you wish to add, and then click 'OK'.
Once completed your old SSDs will be removed, and your new SSDs will be added.
For additional detail please review this link: