Btrfs/snapshots

This page proposes a script to make automatic snapshots with Btrfs filesystem, using btrfs subvolume list-new function to create snapshots only when files have changed, so as to create fewer snapshots.

Note
The script assumes the following layout:

• /mnt/pool - mountpoint for btrfs root volume
• /mnt/pool/volumes/home - user homes volume
• /mnt/pool/snapshots/home - symlink to latest snapshot
• /mnt/pool/snapshots/home_@GMT_2017.02.11-22.00.01 - latest snapshot of home

FILE /mnt/pool/snapshots/snapshot_home.shAutomatic incremental snapshots

#!/bin/bash
d="$(date +%Y.%m.%d-%H.%M.%S)"
echo "Backup started at ${d}"
time="@GMT_${d}"

# Mountpoint
mount="/mnt/pool/volumes/home"

## symlink to most recent snapshot
symlink="/mnt/pool/snapshots/home"
old_snap="$(readlink "/mnt/pool/snapshots/home")"

## New snapshot name
new_snap=${symlink}"_${time}"

## Check for most recent generation ID for most recent snapshot.
## This is used when looking for changed files.
if [[ -d "${old_snap}" ]]; then
        # find-new outputs last generation ID when using a too high value is used for comparing.
        gen_id=$(/sbin/btrfs sub find-new "${old_snap}" 9999999|cut -d " " -f 4)

        # Count changed files.
        count="$(/sbin/btrfs subvolume find-new "${mount}" ${gen_id} | cut -d " " -f 17-1000 | sed '/^$/d'| wc -l)"
        /sbin/btrfs subvolume find-new "${mount}" ${gen_id} | cut -d " " -f 17-1000
        # Create a new snapshot if files have changed.
        if [[ ${count} -gt 0 ]]; then
                /sbin/btrfs subvolume snapshot -r "${mount}" "${new_snap}"

                ## Recreate a symlink to the new snapshot
                rm "${symlink}" && ln -s "${new_snap}" "${symlink}"
        else
                echo "No files changed, skipping creating of a new snapshot"
        fi
else
        echo Something went wrong. Check that symlink $symlink points to a real snapshot
fi
echo "Backup finished at $(date +%Y.%m.%d-%H.%M.%S)"

It is recommended to schedule the /mnt/pool/snapshots/snapshot_home.sh with cron.

FILE /etc/cron.hourly/autosnap.shhourly cron script

#!/bin/bash
/mnt/pool/snapshots/snapshot_home.sh >> /mnt/pool/snapshots/snapshot_home.log 2>&1

Important
btrfs subvolume find-new does not detect all types of changes, for example deleted files. To ensure maximum time difference since deletion of files, then schedule normal time-based snapshots.

For more detailed information on btrfs subvolumes and snapshots see the btrfs wiki.

See also

• Btrfs/System Root Guide - Use the Btrfs filesystem as a collection of subvolumes including one as a system root.
• Btrfs native system root guide - An alternative guide on using a subvolume in a Btrfs filesystem as the system's root.
• Btrbk — a tool for creating incremental snapshots and remote backups of Btrfs subvolumes.
• Snapper — a command-line program to create and manage filesystem snapshots, allowing viewing or reversion of changes.
• Samba shadow copies - Using Samba to expose Shadow Copies as 'Previous Versions' to Windows clients.
• Ext4 — an open source disk filesystem and most recent version of the extended series of filesystems.
• ZFS — a next generation filesystem created by Matthew Ahrens and Jeff Bonwick.