I have installed system on ext4 filesystem on lvm (vg name encrypted, root is called encrypted-root) on luks. When I'm trying to run fstrim /, I get fstrim: /: the discard operation is not supported.

My /etc/crypttab contains

cryptroot  UUID=5ddb7e3a-dcbe-442d-85e8-359e944d0717  none  luks,discard,lvm=encrypted

/etc/lvm/lvm.conf contains

        issue_discards = 1

/etc/initramfs-tools/conf.d/cryptroot contains only


(I used update-initramfs -k all -c to create initramfs).

/etc/default/grub contains


I have tried to manually put rd.luks.options=discard as parameter for linux in grub. I have tried refresh option of cryptsetup utility (cryptsetup --allow-discards refreshdevice), but it does not seem to have one (cryptsetup: Unknown action).

Physical device apparently has TRIM support, when I run fstrim /boot it works (it's same device, just not encrypted).

dmsetup table command does not show allow_discards for cryptroot.

When I boot from USB and manually decrypt (with --allow-discards argument to cryptsetup) and mount root partition, it works. I have tried to use --persistent option, but it said that it couldn't make it persistent.

I'm lost. What should I do to make fstrim / work? Something tells me I should somehow modify boot options in grub, but I'm not sure how. I'm also not sure if line in /etc/crypttab is used at all (I changed it to cryptroot after install, it seems to do nothing even if I change it).

I'm running Linux Mint 19.3.


  • I have followed this guide to encrypt system: link
  • Arch-wiki section about SSDs and dm-crypt
  • possibly relevant: 1, 2

I figured out problem was on encryption level. (You can run dmsetup table as root to see if your device shows allow_discards.)

This fixed the problem for me:

  • Edit /etc/initramfs-tools/conf.d/cryptroot, add discard option for particular device. In my case line looked like this: CRYPTROOT=target=encrypted-root,source=/dev/disk/by-uuid/5ddb7e3a-dcbe-442d-85e8-359e944d0717,discard.
  • Run update-initramfs -k all -c. (Maybe make backup first in case there is something wrong with your config and you don't know about it.)

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