I have never used a setup like that - all I've done is throw an SSD in front of 3 disks. That said, the zfs log and zfs cache command supports mirror disks for

I have never used a setup like that - all I've done is throw an SSD in front of 3 disks. That said, the zfs log and zfs cache command supports mirror disks for log and allows more than 1 cache per pool. Using more than 1 SSD as a cache (and of course, using mirrored logs for safety) would be recommended if you had a production box with this. To repeat, I have never tried ZFS-on-Linux anywhere near a production setup - but if I had to, I would use multiple SSDs just in case one locks up its firmware.