attempting impermanence on testbed machine

utilities/nixos-installers/install_notes.md

@@ -69,6 +69,7 @@ nixos-generate-config --root /mnt
 # @swap options + "noatime"
 # @snapshots options + "compress=zstd" "noatime"
 # @persist options + "compress=zstd"
+#   + neededForBoot = true;
 
 # add Swap device
 swapDevices = [{ 
@@ -132,6 +133,36 @@ In hardware-configuration ensure these are all added:
   };
 ```
 
+### Impermanence BTRFS setup (optional)
+
+```hardware-configuration.nix
+boot.initrd.postResumeCommands = lib.mkAfter ''
+    mkdir /btrfs_tmp
+    mount /dev/ROOT_FILESYSTEM /btrfs_tmp
+    if [[ -e /btrfs_tmp/@root ]]; then
+        mkdir -p /btrfs_tmp/old_roots
+        timestamp=$(date --date="@$(stat -c %Y /btrfs_tmp/root)" "+%Y-%m-%-d_%H:%M:%S")
+        mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp"
+    fi
+
+    delete_subvolume_recursively() {
+        IFS=$'\n'
+        for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
+            delete_subvolume_recursively "/btrfs_tmp/$i"
+        done
+        btrfs subvolume delete "$1"
+    }
+
+    for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +30); do
+        delete_subvolume_recursively "$i"
+    done
+
+    btrfs subvolume create /btrfs_tmp/@root
+    umount /btrfs_tmp
+'';
+
+```
+
 ### Install nixos
 
 `sudo nixos-install`