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Code Issues Releases Wiki Activity

Clarify FAQ regarding backup of virtual machines (#1672)

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enkore 2016-10-02 20:11:34 +02:00 committed by GitHub
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docs/faq.rst
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@ -12,6 +12,39 @@ Yes, the `deduplication`_ technique used by
|project_name| makes sure only the modified parts of the file are stored.
Also, we have optional simple sparse file support for extract.
If you use non-snapshotting backup tools like Borg to back up virtual machines,
then these should be turned off for doing so. Backing up live VMs this way can (and will)
result in corrupted or inconsistent backup contents: a VM image is just a regular file to
Borg with the same issues as regular files when it comes to concurrent reading and writing from
the same file.
For backing up live VMs use file system snapshots on the VM host, which establishes
crash-consistency for the VM images. This means that with most file systems
(that are journaling) the FS will always be fine in the backup (but may need a
journal replay to become accessible).
Usually this does not mean that file *contents* on the VM are consistent, since file
contents are normally not journaled. Notable exceptions are ext4 in data=journal mode,
ZFS and btrfs (unless nodatacow is used).
Applications designed with crash-consistency in mind (most relational databases
like PostgreSQL, SQLite etc. but also for example Borg repositories) should always
be able to recover to a consistent state from a backup created with
crash-consistent snapshots (even on ext4 with data=writeback or XFS).
Hypervisor snapshots capturing most of the VM's state can also be used for backups
and can be a better alternative to pure file system based snapshots of the VM's disk,
since no state is lost. Depending on the application this can be the easiest and most
reliable way to create application-consistent backups.
Other applications may require a lot of work to reach application-consistency:
It's a broad and complex issue that cannot be explained in entirety here.
Borg doesn't intend to address these issues due to their huge complexity
and platform/software dependency. Combining Borg with the mechanisms provided
by the platform (snapshots, hypervisor features) will be the best approach
to start tackling them.
Can I backup from multiple servers into a single repository?
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