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Windows-to-Linux roadmap: Part 8. Backup and recovery
By Chris Walden - 2004-08-03 Page:  1 2 3 4 5

Backing up with dump

Running a backup with dump is fairly straightforward. The following command does a full backup of Linux with all ext2 and ext3 file systems to a SCSI tape device:

dump 0f /dev/nst0 /boot
dump 0f /dev/nst0 /

In this example, our system has two file systems. One for /boot and another for / -- a common configuration. They must be referenced individually when a backup is executed. The /dev/nst0 refers to the first SCSI tape, but in a non-rewind mode. This ensures that the volumes are put back-to-back on the tape.

An interesting feature of dump is its built-in incremental backup functionality. In the example above, the 0 indicates a level 0, or base-level, backup. This is the full system backup that you would do periodically to capture the entire system. On subsequent backups you can use other numbers (1-9) in place of the 0 to change the level of the backup. A level 1 backup would save all of the files that had changed since the level 0 backup was done. Level 2 would backup everything that had changed from level 1 and so on. The same function can be done with tar, using scripting, but it requires the script creator to have a mechanism to determine when the last backup was done. dump has its own mechanism, writing an update file (/etc/dumpupdates) when it performs a backup. The update file is reset whenever a level 0 backup is run. Subsequent levels leave their mark until another level 0 is done. If you are doing a tape-based backup, dump will automatically track multiple volumes.

Skipping files

It is possible to mark files and directories to be skipped by dump. The command for this is chattr, which changes the extended attributes on ext2 and ext3 file systems.

chattr +d

The above command adds a flag to a file to tell dump to skip it when doing a backup.

Restoringwith restore

To restore information saved with dump, the restore command is used. Like tar, dump has the ability to list (-t) and compare archives to current files (-C). Where you must be careful with dump is in restoring data. There are two very different approaches, and you must use the correct one to have predictable results.

Rebuild (-r)

Remember that dump is designed with file systems in mind more than individual files. Therefore, there are two different styles of restoring files. To rebuild a file system, use the -r switch. Rebuild is designed to work on an empty file system and restore it back to the saved state. Before running rebuild, you should have created, formatted, and mounted the file system. You should not run rebuild on a file system that contains files.

Here is an example of doing a full rebuild from the dump that we executed above.

restore -rf /dev/nst0

The above command needs to be run for each file system being restored.

This process could be repeated to add the incremental backups if required.

Extract (-x)

If you need to work with individual files, rather than full file systems, you must use the -x switch to extract them. For example, to extract only the /etc directory from our tape backup, use the following command:

restore -xf /dev/nst0 /etc

Interactive restore (-i)

One more feature that restore provides is an interactive mode. Using the command:

restore -if /dev/nst0

will place you in an interactive shell, showing the items contained in the archive. Typing "help" will give you a list of commands. You can then browse and select the items you wish to be extracted. Bear in mind that any files that you extract will go into your current directory.

dump vs. tar

Both dump and tar have their followings. Both have advantages and disadvantages. If you are running anything but an ext2 or ext3 file system, then dump is not available to you. However, if this is not the case, dump can be run with a minimum of scripting, and has interactive modes available to assist with restoration.

I tend to use tar, because I am fond of scripting for that extra level of control. There are also multi-platform tools for working with .tar files.

Other tools

Virtually any program that can copy files can be used to perform some sort of backup in Linux. There are references to people using cpio and dd for backups. cpio is another packaging utility along the lines of tar. It is much less common. dd is a file system copy utility that makes binary copies of file systems. dd might be used to make an image of a hard drive, similar to using a product like Symantec's Ghost. However, dd is not file based, so you can only restore data to an identical hard drive partition.



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First published by IBM developerWorks


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