My system has been going sluggish lately. After getting suspicious, I decided to check if TRIM was working.

It wasn’t.

How to check and enable

This only applies if you have a LUKS-encrypted root partition. To see if it is correctly set up, run

$ sudo dmsetup table
…
luks-123abcdef-etc: 0 1234567 crypt aes-xts-plain64 000etc000 0 8:2 4096 1 allow_discards
…

If you see allow_discards on every relevant entry (like above), you’re all set. If not, run

$ rpm-ostree kargs --append=rd.luks.options=discard

This applies the ‘discards’ flag at boot time, enabling it for all partitions on the root drive. (If you only want this for some, use rd.luks.options=<LUKS partition ID>=discard). Restart and run fstrim.service. On my machine, it was a difference between light and day.