Update2: systemd now has support for enabling trim on luks partitions by passing in the rd.luks.options=discard option and rebuilding grub config
Update: The latest versions of Fedora now support the discard option in crypttab, not allow-discards.

I have an (unfortunately too small) Samsung 840 Pro in my laptop and it’s been a long time since I’ve re-installed (no time for Korora for months) and I’ve noticed it getting a little sluggish. Most noticeable is long pauses while the drive goes nuts. I figured it was probably time to get some TRIM action on the drive, something I never bothered with before because I didn’t really care.

My setup is reasonably common, I imagine. I have a regular old boot partition and a second encrypted partition which is used as a physical volume for lvm. Hence any and all lv are automatically encrypted. If you’re using encryption, it’s possible that enabling trim could give an attacker insight into what blocks have/haven’t been used, but for me it’s just to make it harder for someone to get my data if I lose the laptop or it’s stolen.

Filesystem
First things first, the file system needs to support trim (ext4 does). If you’re using Fedora 18 you may have to edit your /etc/fstab and add the discard mount option to any partition you want to trim.
/dev/sda1 /boot ext4 defaults,discard 1 2

Under Fedora 19, my non-encrypted, non-lvm /boot partition works with fstrim out of the box (I didn’t have to set the discard mount option), so that’s good.

chris@localhost ~ $ sudo fstrim -v /boot
[sudo] password for chris:
/boot: 407 MiB (426762240 bytes) trimmed


With my / and /home partitions however it’s a different story, I get this:
chris@localhost ~ $ sudo fstrim -v /home
fstrim: /home: FITRIM ioctl failed: Operation not supported

So, problem is that somewhere along the way the discard commands aren’t reaching the device.

I have filesystem, lvm, luks, block layers I guess and I know it’s not the first or the last, so that leaves lvm and luks. Thanks to this post, it was pretty easy to enable on the latter two.

LVM
I edited the /etc/lvm/lvm.conf file and enabled the issue_discards option:
issue_discards = 1


LUKS
Now to ensure that discards are sent to my crypto layer by adding the allow-discards option to /etc/crypttab
luks-blah-blah-blah UUID=blah-blah-blah none allow-discards

Note: Thanks to chesty for pointing out that on Debian and other distros the format of that file and discards option may be different. Check man crypttab for the right option, but it may be something like this:
luks-blah-blah-blah UUID=blah-blah-blah none luks,discard

Initramfs
OK, so config files are in place, no as both of these configs are included in the initramfs, time to rebuild it:
chris@localhost ~ $ sudo dracut --force

Note: For Fedora 18 I had to tell dracut to include the crypttab file, as per this bug report.
chris@localhost ~ $ sudo dracut --force -I /etc/crypttab

Note2: Again, on Debian updating initramfs is different, try the update-initramfs command.

You can confirm that crypttab is in the initramfs with:
chris@localhost ~ $ sudo lsinitrd |grep crypttab

Test
After a reboot, I can test out fstrim again, which now works! (By the way, it’s fast.)
chris@localhost ~ $ time sudo fstrim -v /home
/home: 332.6 MiB (348778496 bytes) trimmed
 
real 0m0.194s
user 0m0.007s
sys 0m0.015s

Cron it
Finally, set this as an hourly cron job and enjoy the benefits.
root@localhost ~ # echo -e "fstrim /\nfstrim /home\nfstrim /boot" > /etc/cron.hourly/fstrim