How can I add a drive to ZFS L2arc cache via the Web UI ?
To add SSD read cache (L2ARC) via the web interface just right-click on the Storage Pool that you want to add SSDs to, then choose "Add Cache Device" and the dialog will appear.
In there you can add SSD write cache devices for the filesystem intent log (ZIL) as well as SSD for read cache (L2ARC).
Note: The SSD hardware cache helps with Random file access, for sequential workloads, all disk I/O will be dependent on the Platter disk speeds.
Having multiple random read or write streams will result in a random access work load and for Platter based RAID configurations this will increase I/O latency and will affect sequential performance.
Please review the link below for additional detail: