Backy is a block-based backup and restore utility for virtual machine images.

Backy is intended to be:

  • space-, time-, and network-efficient
  • trivial to restore
  • reliable.

To achieve this, we rely on:

  • using a copy-on-write filesystem (btrfs, ZFS) as the target filesystem to achieve space-efficiency,
  • using a snapshot-capable main storage for our volumes (e.g. Ceph, LVM, ...) that allows easy extraction of changes between snapshots,
  • leverage proven, existing low-level tools,
  • keep the code-base small, simple, and well-tested.


Full restore

The most important question is: I screwed up – how do I get my data back?

Here’s the fast answer to make a full restore of the most recent backup:

$ cd /srv/backy/my-virtual-machine
$ dd if=last of=/srv/kvm/my-virtual-machine.img bs=4096k

If you like to pick a specific version, it’s only a little more effort:

$ cd /srv/backy/my-virtual-machine
$ backy status
| Date                | ID         |       Size |   Durat | Tags         |
| 2015-11-04 11:09:26 | UT7PkENubw |  60.00 GiB | 845.0 s | weekly,daily |
| 2015-11-05 10:32:03 | fPnbSvEHHy | 264.85 MiB |  88.1 s | daily        |
| 2015-11-06 10:32:03 | cErS5GJ5sL | 172.34 MiB |  84.5 s | daily        |
3 revisions containing 60.43 GiB data (estimated)
$ dd if=fPnbSvEHHymfztN9FuegLQ of=/srv/kvm/my-virtual-machine bs=4096k

Restoring individual files

The image files are exact copies of the data from the virtual disks. You can use regular Linux tools to interact with them:

$ cd /srv/backy/my-virtual-machine
$ ls -l last
lrwxrwxrwx 1 root root  36 Apr 25 10:13 last -> cErS5GJ5sLdsk9L6oCs4ia
$ kpartx -av cErS5GJ5sLdsk9L6oCs4ia
add map loop0p1 (253:9): 0 41934815 linear /dev/loop0 8192
$ mkdir /root/restore
$ mount -o ro /dev/mapper/loop0p1 /root/restore
$ cd /root/restore
$ ls
bin  boot  dev  etc  home  lib  lost+found  media  mnt  opt  proc  root  run
sbin  srv  sys  tmp  usr  var

To clean up:

$ umount /root/restore
$ kpartx -d cErS5GJ5sLdsk9L6oCs4ia

Setting up backy

  1. Create a sufficiently large backup partition using a COW-capable filesystem like btrfs and mount it under /srv/backy.

  2. Create a configuration file at /etc/backy.conf. See man page for details.

  3. Start the scheduler with your favourite init system:

    backy -l /var/log/backy.log scheduler -c /path/to/backy.conf

    The scheduler runs in the foreground until it is shot by SIGTERM.

  4. Set up monitoring using backy check.

  5. Set up log rotation for /var/log/backy.conf and /srv/backy/*/backy.log.

The file paths given above match the built-in defaults, but paths are fully configurable.


Telnet shell

Telnet into localhost port 6023 to get an interactive console. The console can currently be used to inspect the scheduler’s live status.


Backy includes a self-checking facility. Invoke backy check to see if there is a recent revision present for all configured backup jobs:

$ backy check
OK: 9 jobs within SLA

Both output and exit code are suited for processing with Nagios-compatible monitoring systems.

Pluggable backup sources

Backy comes with a number of plug-ins which define block-file like sources:

  • file extracts data from simple image files living on a regular file system.
  • ceph-rbd pulls data from RBD images using Ceph features like snapshots.
  • flyingcircus is an extension to the ceph-rbd source which we use internally on the Flying Circus hosting platform. It uses advanced features like Consul integration.

It should be easy to write plug-ins for additional sources.

Adaptive verification

Backy always verifies freshly created backups. Verification scale depends on the source type: file-based sources get fully verified. Ceph-based sources are verified based on random samples for runtime reasons.

Zero-configuration scheduling

The backy scheduler is intended to run continuously. It will spread jobs according to the configured run intervals over the day. After resuming from an interruption, it will reschedule missed jobs so that SLAs are still kept if possible.

Backup jobs can be triggered at specific times as well: just invoke backy backup manually.