@gnidorah, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @offlinehacker and @bjornfor to be potential reviewers.

Can we use the upstream units included in util-linux? It looks like we're not currently installing them. If we did, we could just say

systemd.packages = [ pkgs.utillinux ];

to get the fstrim units.

I guess a note like "Note, that it won't TRIM whole SSD, but only mounted filesystems. Unused partitions and unused LVM space should be TRIMmed manually." should be added.

Personally, I use an adapted script which takes care of unallocated LVM space.

  periodic_fstrim = { pkgs, ... }: {
    environment.etc."lvm/lvm.conf".text = ''
      devices {
        issue_discards = 1
      }
    '';

    systemd.services.periodic_fstrim = {
      startAt = "Sun 00:00:00";
      path = with pkgs; [ utillinux lvm2 ];
      script = ''
        echo TRIMming mounted filesystems...
        time fstrim -v --all
        echo TRIMming unused LVM space...
        lvm vgs -o vg_name,vg_free_count --noheadings | while read vgname free ; do
		      vgdev="/dev/$vgname/discard"
		      if [ -e "$vgdev" ]; then
			       echo "Logical volume $vgdev does already exist." >&2
			       echo "If it is just a leftover from a previous run then remove it with:" >&2
			       echo "lvm lvremove -f \"$vgdev\"" >&2
		      elif [ "$free" -gt 0 ]; then
			       lvm lvcreate -l100%FREE -n discard "$vgname"
			       cnt=5
			       while [ $cnt -gt 0 ] && ! time lvm lvremove -f "$vgdev" ; do
				       sleep 1
				       cnt=$(($cnt-1))
			       done
			       if [ "$cnt" -eq 0 ]; then
				       echo "Could not remove logical volume $vgdev" >&2
			       fi
		      fi
        done
        echo TRIMming finished
      '';
    };
  };

@edolstra Thank you much! Let's try it.

1. At least for me, in utillinux package, $out in configureFlags is mistakenly not evaluated: https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/util-linux/default.nix#L36

   configuring
   fixing libtool script ./config/ltmain.sh
   configure flags:
   ...
   --with-systemdsystemunitdir=$out/lib/systemd/system/
   
   so units are not shipped. Possible fix pushed, please correct if needed.
2. Besides systemd.packages = [ pkgs.utillinux ]; I heard that an additional explicit wantedBy is needed in NixOS to deal dependencies, or am I wrong?

@danbst Interesting what to do with EFI partition, since fstrim still not supported by vfat. Distros what I've seen do nothing (don't even use discard flag for it).

I now get cycle detected error. Suggestions?

Regarding the cycle, where do the units get installed? If they're installed in the same output as the binaries, you shouldn't get a cycle. Then you also don't need to patch out the ExecStart paths and don't need to override ExecStart in the module. (The usefulness of the arguments option is debatable IMHO.)

Yes, a wantedBy is needed to pull in the timer.

@edolstra Thanks! Haven't meet split derivations before, moved units installation from out to bin and removed arguments option in module.

Seems to work here. BTW, do these systemd-related configure flags also need to be added to crossAttrs.preConfigure?

Given that this changes util-linux, I expect this will be a large rebuild. Perhaps should go via staging.

Merged to staging, thanks!

@edolstra Thank you!

@bjornfor Installed fstrim.timer from utillinux already do this.

@gnidorah: I don't understand what that means.

@bjornfor
In short: upstream does this.
This a996fe8#diff-fab06f256b6956e04e4806ab5b9dabfbR34 says to use units from utillinux package.
And fstrim.timer unit from utillinux package defines Persistent=true like you suggested https://github.com/karelzak/util-linux/blob/stable/v2.29/sys-utils/fstrim.timer#L8