fstrim does not work

Update: I wrote the answer below before seeing you were using ext2. I keep the answer for reference in case anyone else has a similar problem because of LUKS or LVM.

For your case however the anser is different: ext2 does not support trimming, only ext4 or btrfs do.

----- original answer -----

Make sure to activate trim support on all filesystem abstraction layers. In particular, LVM and / or dm-crypt come to mind here!

1. Activate trim support in lvm: /etc/lvm/lvm.conf

   issue_discards = 1
   

2. Activate trim support in dm-crypt/luks: /etc/crypttab

   sda1_crypt UUID=<YOUR_DISKS_UUID> none luks,**discard**
   

   (adapt to your UUID and partition)

3. write changes to your initrd:

   sudo update-initramfs -u
   

4. reboot

You need to run fstrim as root. Try:

sudo fstrim /