microvm-defaults: disable auto optimise, use mkDefault

config/default.nix

@@ -27,8 +27,6 @@
       "net.ipv4.tcp_congestion_control" = "bbr";
     };
     tmp.cleanOnBoot = true;
-    # recommend to turn off, only on by default for backwards compatibility
-    zfs.forceImportRoot = false;
   };
 
   c3d2 = {
@@ -158,7 +156,7 @@
       trusted-public-keys = [
         "nix-cache.hq.c3d2.de:KZRGGnwOYzys6pxgM8jlur36RmkJQ/y8y62e52fj1ps="
       ];
-      stalled-download-timeout = 60; # in case hydra is not reachable fail faster
+      stalled-download-timeout = 30; # in case hydra is not reachable fail faster
       # don't self feed hydra
       substituters = lib.mkIf (config.networking.hostName != "hydra") (
         lib.mkBefore [ "https://nix-cache.hq.c3d2.de" ]
@@ -187,7 +185,7 @@
     tmux = {
       enable = true;
       historyLimit = 50000;
-      extraConfig = ''
+      extraConfig = /* tmux */ ''
         # mouse control
         set -g mouse on
 

hosts/knot/default.nix

@@ -33,6 +33,23 @@
     };
   };
 
+  # required for CI
+  microvm = let
+    writableStoreOverlayImage = "/var/tmp/nix-store-overlay.img";
+  in {
+    preStart = ''
+      # Discard old writable store overlay
+      rm -f "${writableStoreOverlayImage}"
+    '';
+    volumes = [ {
+      image = writableStoreOverlayImage;
+      mountPoint = config.microvm.writableStoreOverlay;
+      size = 1 * 1024;
+    } ];
+    writableStoreOverlay = "/nix/.rw-store";
+  };
+  nix.enable = true;
+
   services.knot = {
     enable = true;
     keyFiles = [ config.sops.secrets."knot/keyFile".path ];

modules/microvm-defaults.nix

@@ -37,8 +37,9 @@
 
   # nix store is mounted read only
   nix = {
-    enable = false;
+    enable = lib.mkDefault false;
     gc.automatic = false;
+    optimise.automatic = false;
   };
 
   systemd.tmpfiles.rules = [