.claude/tasks/0014-nix-store-housekeeping.md

@@ -10,9 +10,21 @@ Retained boot configurations are capped at 15. Each generation stores a kernel a
 
 ## Acceptance criteria
 
-- [ ] Automatic garbage collection is enabled weekly, deleting generations older than 30 days
-- [ ] Store optimisation is scheduled weekly, rather than performed at build time
-- [ ] Retained boot configurations are capped at 15
-- [ ] These are declared as plumbing in the shared base config, so every future `Host` inherits them
-- [ ] `nix flake check` builds the `neogaia` toplevel
-- [ ] Manual confirmation after a rebuild: the collection and optimisation timers exist and are scheduled
+- [x] Automatic garbage collection is enabled weekly, deleting generations older than 30 days
+- [x] Store optimisation is scheduled weekly, rather than performed at build time
+- [x] Retained boot configurations are capped at 15
+- [x] These are declared as plumbing in the shared base config, so every future `Host` inherits them
+- [x] `nix flake check` builds the `neogaia` toplevel
+- [x] Manual confirmation after a rebuild: the collection and optimisation timers exist and are scheduled
+
+## Implementation Notes
+
+The two schedules are named days rather than the bare `weekly` keyword.
+systemd expands `weekly` to `Mon *-*-* 00:00:00`, which would have started collection and deduplication at the same instant every week, leaving `nix-optimise` hard-linking paths `nix-gc` was concurrently deleting.
+Collection now runs `Mon 03:15` and optimisation `Thu 03:45`, which keeps both weekly and keeps them apart.
+
+The boot configuration cap sits in the shared base as the task asks, and is inert rather than an error on a host that does not use systemd-boot.
+A future host on another bootloader therefore inherits no cap, which is the one place the "every future host inherits them" promise does not reach.
+
+Confirmed on `neogaia` after a switch: `systemctl list-timers 'nix-*'` lists both units, `nix-optimise` next on Thursday and `nix-gc` next on Monday, each `Persistent=true` so a suspended laptop catches up on a missed firing.
+`/boot` reports 2 GiB with 113 MiB used, so the cap of 15 sits against the enlarged partition it assumes.

system/default.nix

@@ -58,6 +58,26 @@ in
     nix.settings.extra-trusted-public-keys = [
       "nyx-cache.chaotic.cx:dJxTrgMC3V3cFfyIiBQDQorG6k1LsqurH/srpMSq7qk="
     ];
+
+    # A month of generations is kept, because on a rolling channel with a
+    # third-party kernel an old generation is a known-good system to boot when
+    # an update breaks something.
+    nix.gc.automatic = true;
+    nix.gc.dates = "Mon 03:15";
+    nix.gc.options = "--delete-older-than 30d";
+
+    # Deduplication runs on a timer, off the rebuild path, so it never adds
+    # latency to a `nixos-rebuild switch`.
+    # It falls on a different day from collection, so the two never contend.
+    nix.optimise.automatic = true;
+    nix.optimise.dates = [ "Thu 03:45" ];
+
+    # The EFI system partition holds a kernel and an initrd per entry at roughly
+    # 70 MiB apiece, and is fixed in size.
+    # An exhausted one fails at bootloader installation, after the build has
+    # already succeeded.
+    boot.loader.systemd-boot.configurationLimit = 15;
+
     environment.systemPackages = [ pkgs.git ];
 
     # Caps Lock is a second Escape; Shift+Caps Lock still toggles Caps Lock.