Limit journal size

system/shared/services/fstrim.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }: let
+  inherit (lib.modules) mkIf;
+in {
+  # if lvm is enabled, then tell it to issue discards
+  # (this is good for SSDs and has almost no downsides on HDDs, so
+  # it's a good idea to enable it unconditionally)
+  environment.etc."lvm/lvm.conf".text = mkIf config.services.lvm.enable ''
+    devices {
+      issue_discards = 1
+    }
+  '';
+
+  # discard blocks that are not in use by the filesystem, good for SSDs
+  services.fstrim = {
+    # we may enable this unconditionally across all systems becuase it's performance
+    # impact is negligible on systems without a SSD - which means it's a no-op with
+    # almost no downsides aside from the service firing once per week
+    enable = true;
+
+    # the default value, good enough for average-load systems
+    interval = "weekly";
+  };
+
+  # tweak fstim service to run only when on AC power
+  # and to be nice to other processes
+  # (this is a good idea for any service that runs periodically)
+  systemd.services.fstrim = {
+    unitConfig.ConditionACPower = true;
+
+    serviceConfig = {
+      Nice = 19;
+      IOSchedulingClass = "idle";
+    };
+  };
+}