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