BTRFS Snapshot

BTRFS Snapshot is a simple set of scripts and systemd units for taking and managing BTRFS snapshots. There actual project like Snapper and yabsnap, but this one does exactly what I need reliably and predictably with no frills, GUIs, etc.

Installation

You can install this project with a standard make & make install invocation:

make
sudo make install

You should then enable the various units. E.g., to backup your home directory on a schedule, you can run:

systemctl enable --user --now "$(systemd-escape "$HOME" --template btrfs-snapshot@.timer)"
systemctl enable --user --now "$(systemd-escape "$HOME" --template btrfs-snapshot-cleanup@.timer)"

(assuming your home directory is on its own BTRFS subvolume)

Interval

Snapshots are taken to $SUBVOL/.backups every hour by the btrfs-snapshot@.timer unit. The interval can be configured by modifying/replacing the timer unit.

Snapshot Cleaning

Snapshots are cleaned when they’re taken:

  • All files specified by $SUBVOL/.backupignore are removed. This file lists files rooted at $SUBVOL with no patterns or globbing for now.
  • All directories containing a CACHEDIR.TAG file are deleted.
  • All git repositories are cleared of ignored files/directories. I.e., git clean -Xd is run in every git repository.

Snapshot Pruning

If the snapshot cleanup unit (btrfs-snapshot-cleanup@.timer) is enabled, snapshots will be pruned once a day as follows:

  • All snapshots from the last 24 hours are kept.
  • One snapshot is kept for each day of the last month.
  • One snapshot is kept per week of the last year.
  • One snapshot is kept per month forever.

Emacs Integration

If you want to explore a file’s history, I recommend the snapshot-timeline package (available on MELPA). Configure it with the following backend and it should “just work” (automatically locating the most “relevant” .backups directory.

(defun snapshot-backend-btrfs (filename)
  (when-let* ((base (expand-file-name (locate-dominating-file filename ".backups/")))
              (rfname (string-remove-prefix base filename))
              (bdir (concat base ".backups/"))
              (files (directory-files bdir nil (rx bos (not ?.))))
              (count 0))
    (seq-keep
     (lambda (d)
       (when-let* ((bname (concat bdir d "/" rfname))
                   ((file-exists-p bname))
                   (date (ignore-errors
                           (time-convert (encode-time
                                          (parse-time-string d))
                                         'list))))
         (incf count)
         (make-snapshot :id count :name (number-to-string count) :file bname :date date)))
     files)))

(setopt snapshot-timemachine-snapshot-finder #'snapshot-backend-btrfs)