Incremental Backup using tar

[vinayakk]

GNU tar currently offers two options for handling incremental backups: ‘--listed-incremental=snapshot-file’ (‘-g snapshot-file’) and ‘--incremental’ (‘-G’).

The option ‘--listed-incremental’ instructs tar to operate on an incremental archive with additional metadata stored in a standalone file, called a snapshot file.

To create an incremental backup, you would use ‘--listed-incremental’ together with ‘--create’.

For example:
    

$ tar --create \
           --file=archive.1.tar \
           --listed-incremental=/var/log/usr.snar \
           /usr

This will create in ‘archive.1.tar’ an incremental backup of the ‘/usr’ file system, storing additional metadata in the file ‘/var/log/usr.snar’. If this file does not exist, it will be created. The created archive will then be a level 0 backup.

Otherwise, if the file ‘/var/log/usr.snar’ exists, it determines which files are modified. In this case only these files will be stored in the archive. Suppose, for example, that after running the above command, you delete file ‘/usr/doc/old’ and create directory ‘/usr/local/db’ with the following contents:
    

$ ls /usr/local/db
/usr/local/db/data
/usr/local/db/index

Some time later you create another incremental backup. You will then see:
    

$ tar --create \
           --file=archive.2.tar \
           --listed-incremental=/var/log/usr.snar \
           /usr


tar: usr/local/db: Directory is new
usr/local/db/
usr/local/db/data
usr/local/db/index

The created archive ‘archive.2.tar’ will contain only these three members. This archive is called a level 1 backup. Notice that ‘/var/log/usr.snar’ will be updated with the new data, so if you plan to create more ‘level 1’ backups, it is necessary to create a working copy of the snapshot file before running tar. The above example will then be modified as follows:
    

$ cp /var/log/usr.snar /var/log/usr.snar-1

$ tar --create \
           --file=archive.2.tar \
           --listed-incremental=/var/log/usr.snar-1 \
           /usr

To extract from the incremental dumps, use ‘--listed-incremental’ together with ‘--extract’ option.
In this case, tar does not need to access snapshot file, since all the data necessary for extraction are stored in the archive itself. So, when extracting, you can give whatever argument to ‘--listed-incremental’, the usual practice is to use ‘--listed-incremental=/dev/null’. Alternatively, you can use ‘--incremental’, which needs no arguments. In general, ‘--incremental’ (‘-G’) can be used as a shortcut for ‘--listed-incremental’ when listing or extracting incremental backups.

When extracting from the incremental backup GNU tar attempts to restore the exact state the file system had when the archive was created. In particular, it will delete those files in the file system that did not exist in their directories when the archive was created. If you have created several levels of incremental files, then in order to restore the exact contents the file system had when the last level was created, you will need to restore from all backups in turn. Continuing our example, to restore the state of ‘/usr’ file system,

$ tar --extract \
           --listed-incremental=/dev/null \
           --file archive.1.tar
$ tar --extract \
           --listed-incremental=/dev/null \
           --file archive.2.tar

To list the contents of an incremental archive, use ‘--list’. To obtain more information about the archive, use ‘--listed-incremental’ or ‘--incremental’ combined with two ‘--verbose’ options.
    

$ tar --list --incremental --verbose --verbose archive.tar