DATAPUISTOKEMISTI

Depending on why you're encrypting your SSDs it might be acceptable to enable TRIM (aka discard). With this configuration observing raw content on SSDs will reveal which blocks are used and which aren't. Doesn't matter for me, but might not be same for you.

My original plan was to boot from ZFS, but combining it with encryption and especially with mirrored disks turned out to be too troublesome. Yes, I did get it working and booting, but resulting configuration required manual fixing each time new kernel was installed. And don't even think about doing upgrade to next Ubuntu version.