made borg work with lvm

modules/borg.nix

@@ -1,6 +1,7 @@
 { config, pkgs, ... }:
 let
   backuppath = "/backup/snapshot";
+  borgHook = toString ./borgHook.sh;
 in
 {
   services.borgbackup.jobs.default = {
@@ -27,11 +28,8 @@ in
     environment.BORG_RSH = "ssh -o 'StrictHostKeyChecking=no' -i /root/.ssh/id_borg";
     extraCreateArgs = "--verbose --stats";
     startAt = "00:10";
-    preHook = ''if [[ -d ${backuppath} ]]; then
-        btrfs subvolume delete ${backuppath}
-      fi
-      btrfs subvolume snapshot / ${backuppath}''; #create snapshot for consistent filesystem
-    postCreate = "btrfs subvolume delete ${backuppath}"; #delete snapshot again
+    preHook = "${borgHook} pre ${backuppath}"; #create snapshot for consistent filesystem
+    postCreate = "${borgHook} post ${backuppath}"; #delete snapshot again
   };
 
   systemd.services.borgbackup-job-default = {

modules/borgHook.sh

@@ -0,0 +1,31 @@
+#!/run/current-system/sw/bin/bash
+hooktype="$1"
+backuppath="$2"
+fstype="$(df --output=fstype / | sed 1d)"
+function dellvm {
+    umount ${backuppath}
+    lvremove -y "$1"
+}
+function delbtrfs {
+    btrfs subvolume delete ${backuppath}
+}
+if [[ "$fstype" == "btrfs" ]]; then
+    if [[ -d ${backuppath} ]]; then
+        delbtrfs
+    fi
+    if [[ "$hooktype" == "pre" ]]; then
+        btrfs subvolume snapshot / ${backuppath}
+    fi
+else
+    snapname=nixos_snap
+    volume_group="$(mount | grep 'on / ' | cut -f 1 -d' ')"
+    vgname="$(lvs --noheadings "$volume_group" -o vg_name | tr -d ' ')"
+    snappath="/dev/mapper/${vgname}-${snapname}"
+    if [[ -b "$snappath" ]]; then
+        dellvm "$snappath"
+    fi
+    if [[ "$hooktype" == "pre" ]]; then
+        lvcreate -l50%FREE -s -n "$snapname" "$volume_group"
+        mount "$snappath" -o ro /backup/snapshot
+    fi
+fi