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4081 - Add note about files changing during backup (#4116)

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Add note about files changing during backup
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James Vasile 2018-11-01 16:11:05 -04:00 committed by TW
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docs/quickstart.rst
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@ -95,6 +95,46 @@ borg.
If you need to access a local repository from different users, you can use the
same method by using ssh to borg@localhost.
Important note about files changing during the backup process
-------------------------------------------------------------
Borg does not do anything about the internal consistency of the data
it backs up. It just reads and backs up each file in whatever state
that file is when Borg gets to it. On an active system, this can lead
to two kinds of inconsistency:
- By the time Borg backs up a file, it might have changed since the backup process was initiated
- A file could change while Borg is backing it up, making the file internally inconsistent
If you have a set of files and want to ensure that they are backed up
in a specific or consistent state, you must take steps to prevent
changes to those files during the backup process. There are a few
common techniques to achieve this.
- Avoid running any programs that might change the files.
- Snapshot files, filesystems, container storage volumes, or logical volumes. LVM or ZFS might be useful here.
- Dump databases or stop the database servers.
- Shut down virtual machines before backing up their images.
- Shut down containers before backing up their storage volumes.
For some systems Borg might work well enough without these
precautions. If you are simply backing up the files on a system that
isn't very active (e.g. in a typical home directory), Borg usually
works well enough without further care for consistency. Log files and
caches might not be in a perfect state, but this is rarely a problem.
For databases, virtual machines, and containers, there are specific
techniques for backing them up that do not simply use Borg to backup
the underlying filesystem. For databases, check your database
documentation for techniques that will save the database state between
transactions. For virtual machines, consider running the backup on
the VM itself or mounting the filesystem while the VM is shut down.
For Docker containers, perhaps docker's "save" command can help.
Automating backups
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