mkdir /btrfs_tmp
mount /dev/root_vg/root /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