Trim on LUKS encrypted LVM root volumes

I noticed that fstrim wasn’t working on my new Ubuntu 24.04 desktop installation, failing with “fstrim: /: the discard operation is not supported”. In that setup, Ubuntu is installed into an LUKS encrypted LVM volume, so the ext4 file system, LVM and LUKS need to play nicely together for flash discard to work.

That’s how I got it working:

sudo su
Edit /etc/fstab, find where the root volume is mounted (for me that was something like /dev/disk/by-id/dm-uuid-LVM-xyz) and change the mount options to “defaults,discard,errors=remount-ro”
Edit /etc/crypttab and add “,discards” to the options. For me, after editing, the options were “luks,discard”
update-initramfs -c -k all
Reboot

After rebooting, “fstrim -v /” should show that discard worked and “dmsetup table” should show an “allow_discard” flag for the device.