VMs have already TRIM enabled by default, but dom0 doesn’t. There are some security implications (read for example ​this article), but IMO not very serious.

To enable TRIM in dom0 you need:

1. Get your LUKS device UUID:

ls /dev/mapper/luks-*

1. Add entry to /etc/crypttab (replace luks-<UUID> with the device name and the <UUID> with UUID alone):

luks-<UUID> UUID=<UUID> none allow-discards

1. Add rd.luks.allow-discards=1 to kernel cmdline (/etc/default/grub, GRUB_CMDLINE_LINUX line)
2. Rebuild grub config (grub2-mkconfig -o /boot/grub2/grub.cfg)
3. Rebuild initrd in hostonly mode:

dracut -H -f

1. Add “discard” option to /etc/fstab for root device
2. Reboot the system, verify that allow-discards is really enabled (dmsetup table)

There is a ​bug affecting allow-discards option, once it will be fixed, first two steps will be no longer needed.