nixos: Prevent redundant fstrim

The fstrim service, when enabled, by default trims all mounts listed in
fstab (as long as the underlying device supports discard), including
bind mounts. With impermanence, this can lead to a significant number of
redundant trim operations, since the persistent volume is trimmed once
for itself and then again once per persistent directory each. On my
system, this leads to fstrim.service taking about 10 minutes, during
which the system usability is somewhat degraded. In addition, it may be
desired to disable trim for a certain filesystem entirely, for example
when using btrfs with discard=async.

Mark our bind mounts with the X-fstrim.notrim option, which is ignored
by mount, but instructs fstrim to skip the mount point in question. The
persistent filesystem is still trimmed properly, since it has to be
listed in fstab itself as well.

nixos.nix

@@ -56,7 +56,7 @@ let
     value = {
       device = concatPaths [ persistentStoragePath dirPath ];
       noCheck = true;
-      options = [ "bind" ]
+      options = [ "bind" "X-fstrim.notrim" ]
         ++ optional hideMount "x-gvfs-hide";
       depends = [ persistentStoragePath ];
     };