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:

  • space-efficient storages (CoW filesystems, content-hashed chunking)
  • using a snapshot-capable source for our volumes (i.e. Ceph RBD) that allows easy extraction of changes between snapshots,
  • leverage proven, existing low-level tools,
  • keep the code-base small, simple, and well-tested.

We also have a few ground rules for the implementation:

  • VM data is stored self-contained on the filesystem and can be moved between servers using regular FS tools like copy, rsync, or such.
  • No third party daemons are required to interact with backy: no database server. The scheduler daemon is only responsible for scheduling and simply calls regular CLI commands to perform a backup. Backy may interact with external daemons like Ceph or Consul, depending on the source storage implementation.


Full restore

Check which revision to restore:

$ backy -b /srv/backy/<vm> status

Maybe set up the Ceph environment - depending on your configuration:

$ export CEPH_ARGS="--id $HOSTNAME"

Restore the full image through a Pipe:

$ backy restore -r <revision> - | rbd import - <pool>/<rootimage>

Restoring individual files

Backy provides an NBD server to access backups through a mountable device:

$ cd /srv/backy/$vm
$ backy nbd-server

In a different shell you can now mount this:

$ nbd-client -N <revision> localhost 9000 /dev/nbd0
$ mkdir  -p /mnt/restore/<vm>
$ mount -r /dev/nbd0p1 /mnt/restore/<vm>

When done:

$ umount /mnt/restore/<vm>
$ nbd-client -d /dev/nbd0

Also stop the nbd-server with Ctrl-C.

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.


Backy is designed to use all of the available storage and network bandwidth by running several instances in parallel. The backing storage must be prepared for this kind of (mixed) load. Finding optimal settings needs a bit of experimentation given that hardware and load profiles differ from site to site. The following section contains a few points to start off.

Storage backend

If the backing storage is a RAID array, its stripe size should be aligned with the filesystem. We have made good experiences with 256k stripes. Also check for 512B/4K block misalignments on HDDs. We're using it usually with RAID-6 and have seen reasonable performance with both hardware and software RAID.


We generally recommend XFS since it provides a high degree of parallelism and is able to handle very large directories well.

Note that the standard cfq I/O scheduler is not a good pick for highly parallel bulk I/O on multiple drives. Use deadline or noop.


Since backy performs a lot of metadata operations, make sure that inodes and dentries are not evicted from the VFS cache too early. We found that lowering the vm.vfs_cache_pressure sysctl can make quite a difference in total backup performance. We're currently getting good results setting it to 10. You may also want to increase vm.min_free_kbytes to avoid page allocation errors on 10 GbE network interfaces.