+#!/bin/bash
+
+set -eu
+
+function snap {
+ unsnap
+
+ mkdir -p /a
+
+ dispatch snap < /etc/fstab
+}
+
+function unsnap {
+ tac /etc/fstab | dispatch unsnap
+
+ if [ -d "/a" ]; then
+ rmdir /a
+ fi
+}
+
+function dispatch {
+ local action="" snaptype=""
+ local dev="" mnt="" fstype="" opts="" dump=""
+
+ while read -r dev mnt fstype opts dump pass; do
+ snaptype="$(get_snaptype "$fstype" "$dump")"
+ if [ -z "$snaptype" ]; then
+ continue
+ fi
+ action="${1}_${snaptype}"
+ echo "$action $mnt"
+ "$action" "$dev" "$mnt" "$opts"
+ done
+}
+
+function get_snaptype {
+ local fstype="$1" dump="$2"
+ if [ "$dump" = "1" ]; then
+ echo bind
+ elif [ "$dump" = "2" ] && [ "$fstype" = "btrfs" ]; then
+ echo btrfs
+ elif [ "$dump" = "2" ]; then
+ echo lvm
+ fi
+}
+
+function snap_bind {
+ local mnt="$2"
+ mount --bind "${mnt}" "/a${mnt}"
+}
+
+function unsnap_bind {
+ local snapmnt="/a$2"
+ if mountpoint -q "$snapmnt"; then
+ umount "$snapmnt"
+ fi
+}
+
+function snap_lvm {
+ local dev="$1" mnt="$2"
+ local lvname="" vgname=""
+ read -r lvname vgname _ < <(lvdisplay --noheadings -C "$dev")
+ echo "snapshot ${vgname}/${lvname}"
+ lvcreate -L1G --snapshot --name "${lvname}snap" "${vgname}/${lvname}"
+ mount -o ro "${dev}snap" "/a${mnt}"
+}
+
+function unsnap_lvm {
+ local snapdev="${1}snap"
+ local snapmnt="/a$2"
+ if mountpoint -q "$snapmnt"; then
+ umount "$snapmnt"
+ fi
+ if [ -b "$(readlink -f "$snapdev")" ]; then
+ lvremove -f "${dev}snap"
+ fi
+}
+
+function snap_btrfs {
+ local mnt="$2" opts="$3" snapdir=""
+
+ snapdir="$(get_btrfs_snapshot_dir "$opts" "$mnt")"
+ btrfs subvolume snapshot "$mnt" "$snapdir"
+
+ mount --bind "$snapdir" "/a${mnt}"
+}
+
+function unsnap_btrfs {
+ local mnt="$2" opts="$3" snapdir=""
+
+ local snapmnt="/a$2"
+ if mountpoint -q "$snapmnt"; then
+ umount "$snapmnt"
+ fi
+
+ snapdir="$(get_btrfs_snapshot_dir "$opts" "$mnt")"
+ if [ -d "$snapdir" ]; then
+ btrfs subvolume delete "$snapdir"
+ fi
+}
+
+function get_btrfs_snapshot_dir {
+ local opts="$1" mnt="$2" subvol=""
+ subvol="$(echo "$opts" | sed -nE 's/^.*\bsubvol=([^,]+)\b.*$/\1/p')"
+ if [ -z "$subvol" ]; then
+ echo "Unknown subvol for mountpoint: $mnt"
+ exit 1
+ fi
+ echo "/$(hostname -s)/${subvol}-snap"
+}
+
+if [ "$(id -u)" -ne 0 ]; then
+ echo "Must be root"
+ exit 1
+fi
+
+if [ "${1:-}" = "off" ]; then
+ unsnap
+elif [ "${1:-}" = "on" ]; then
+ snap
+else
+ echo "Usage: $0 on|off"
+ exit 1
+fi