fix: automatic garbage collection

NixOS.md

@@ -12,14 +12,17 @@ This document contains general notes about NixOS that are independent of my NixO
   * [niv: Dependency management](#niv-dependency-management)
     * [Add Home Manager with niv](#add-home-manager-with-niv)
     * [Add NUR with niv](#add-nur-with-niv)
-  * [Garbage collection](#garbage-collection)
   * [System information](#system-information)
   * [Search for packages](#search-for-packages)
   * [Search for options](#search-for-options)
   * [Search which package owns a file](#search-which-package-owns-a-file)
   * [List files of package](#list-files-of-package)
   * [Package runtime dependency report](#package-runtime-dependency-report)
-  * [Compare two versions of NixOS system profile](#compare-two-versions-of-nixos-system-profile)
+  * [Nix profiles](#nix-profiles)
+    * [List system profiles](#list-system-profiles)
+    * [Garbage collection - Keep most recent X system profiles](#garbage-collection---keep-most-recent-x-system-profiles)
+    * [Garbage collection - Delete system profiles older than X days](#garbage-collection---delete-system-profiles-older-than-x-days)
+    * [Compare two system profiles](#compare-two-system-profiles)
   * [NixOS configuration debugging](#nixos-configuration-debugging)
   * [Show Nix configuration](#show-nix-configuration)
   * [Evaluate NixOS configuration to JSON](#evaluate-nixos-configuration-to-json)
@@ -144,25 +147,6 @@ niv add nix-community/home-manager -n home-manager -b release-23.05
 niv add nix-community/NUR -n NUR
 ```
 
-## Garbage collection
-
-* https://nixos.org/manual/nix/stable/package-management/garbage-collection.html
-* https://discourse.nixos.org/t/why-doesnt-nix-collect-garbage-remove-old-generations-from-efi-menu/17592/4
-
-This is automated in [base.nix](modules/base.nix) with the `nix.gc` option.
-
-Run manually for all profiles:
-
-```shell
-sudo nix-collect-garbage --delete-older-than 14d
-```
-
-Remove leftover EFI entries of removed generations:
-
-```shell
-sudo /run/current-system/bin/switch-to-configuration boot
-```
-
 ## System information
 
 ```shell
@@ -476,7 +460,43 @@ runtime dependencies of firefox-121.0.1:
  - libXau-1.0.11 (no license) maintained by nobody
 ```
 
-## Compare two versions of NixOS system profile
+## Nix profiles
+
+### List system profiles
+
+```shell
+sudo nix-env --list-generations --profile /nix/var/nix/profiles/system
+```
+
+### Garbage collection - Keep most recent X system profiles
+
+A NixOS config example is given in [base.nix](modules/base.nix).
+
+```shell
+sudo nix-env --delete-generations +7 --profile /nix/var/nix/profiles/system
+```
+
+### Garbage collection - Delete system profiles older than X days
+
+* https://nixos.org/manual/nix/stable/package-management/garbage-collection.html
+* https://discourse.nixos.org/t/why-doesnt-nix-collect-garbage-remove-old-generations-from-efi-menu/17592/4
+
+A NixOS config example is given in [base.nix](modules/base.nix).
+
+Run manually for all profiles:
+
+```shell
+sudo nix-collect-garbage --delete-older-than 14d
+```
+
+Remove leftover EFI entries of removed generations:
+
+```shell
+sudo /run/current-system/bin/switch-to-configuration boot
+```
+
+
+### Compare two system profiles
 
 Get latest system profile. This is the profile (usually) being active after booting the system:
 

modules/base-gc.nix

@@ -0,0 +1,41 @@
+{ config, pkgs, ... }:
+{
+  # Based on https://github.com/NixOS/nixpkgs/blob/nixos-23.11/nixos/modules/services/misc/nix-gc.nix
+
+  assertions = [{
+    assertion = config.nix.enable;
+    message = ''nix-gc-7 requires nix.enable'';
+  }];
+
+  systemd.timers."nix-gc-7" = {
+    wantedBy = [ "timers.target" ];
+    partOf = [ "nix-gc-7.service" ];
+    timerConfig = {
+      OnCalendar = "weekly";
+      Persistent = true;
+
+      AccuracySec = "2d";
+      RandomizedDelaySec = "1d";
+    };
+  };
+
+  systemd.services."nix-gc-7" = {
+    description = "Keep only the most recent 7 system generations";
+
+    conflicts = [ "shutdown.target" "sleep.target" ];
+    before = [ "shutdown.target" "sleep.target" ];
+
+    serviceConfig = {
+      Type = "oneshot";
+      PrivateTmp = true;
+
+      Nice = 19;
+      IOSchedulingClass = "idle";
+    };
+
+    script = ''
+      set -eu -o pipefail
+      ${config.nix.package.out}/bin/nix-env --delete-generations +7 --profile /nix/var/nix/profiles/system
+    '';
+  };
+}

modules/base.nix

@@ -74,18 +74,16 @@
 
   nix.settings.auto-optimise-store = true;
 
+  # Keep only most recent 7 system generations
+  imports = [ ./base-gc.nix ];
+  #
+  # Alternatively: Delete generations older than 7 days
   # https://nixos.wiki/wiki/Storage_optimization#Automation
-  nix.gc = {
+  #nix.gc = {
-    automatic = true;
+  #  automatic = true;
-    dates = "weekly";
+  #  dates = "weekly";
-    # Options given to nix-collect-garbage.
+  #  options = "--delete-older-than 7d";
-    # They are documented here: https://nixos.org/manual/nix/stable/command-ref/nix-collect-garbage
+  #};
-    # The `--delete-older-than <period` option refers the manual for `nix-env --delete-generations <period>`: https://nixos.org/manual/nix/stable/command-ref/nix-env/delete-generations#generations-time
-    # There, two meanings of <period> are described. The second one is:
-    #   +<number>: Keep the last number generations, along with any newer than current.
-    # TODO: +7 does currently not work. There is an open issue: https://github.com/NixOS/nixpkgs/issues/282884
-    options = "--delete-older-than +7";
-  };
 
   # Delete all files in /tmp during boot.
   boot.tmp.cleanOnBoot = true;