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12.4. Simple backups

A simple backup scheme is to back up everything once, then back up everything that has been modified since the previous backup. The first backup is called a full backup, the subsequent ones are incremental backups. A full backup is often more labourious than incremental ones, since there is more data to write to the tape and a full backup might not fit onto one tape (or floppy). Restoring from incremental backups can be many times more work than from a full one. Restoration can be optimised so that you always back up everything since the previous full backup; this way, backups are a bit more work, but there should never be a need to restore more than a full backup and an incremental backup.

If you want to make backups every day and have six tapes, you could use tape 1 for the first full backup (say, on a Friday), and tapes 2 to 5 for the incremental backups (Monday through Thursday). Then you make a new full backup on tape 6 (second Friday), and start doing incremental ones with tapes 2 to 5 again. You don't want to overwrite tape 1 until you've got a new full backup, lest something happens while you're making the full backup. After you've made a full backup to tape 6, you want to keep tape 1 somewhere else, so that when your other backup tapes are destroyed in the fire, you still have at least something left. When you need to make the next full backup, you fetch tape 1 and leave tape 6 in its place.

If you have more than six tapes, you can use the extra ones for full backups. Each time you make a full backup, you use the oldest tape. This way you can have full backups from several previous weeks, which is good if you want to find an old, now deleted file, or an old version of a file.

12.4.1. Making backups with tar

A full backup can easily be made with tar:
# tar --create --file /dev/ftape 
/usr/src
tar: Removing leading / from absolute path names in 
the archive
#
The example above uses the GNU version of tar and its long option names. The traditional version of tar only understands single character options. The GNU version can also handle backups that don't fit on one tape or floppy, and also very long paths; not all traditional versions can do these things. (Linux only uses GNU tar.)
If your backup doesn't fit on one tape, you need to use the --multi-volume (-M) option:
# tar -cMf /dev/fd0H1440 
/usr/src
tar: Removing leading / from absolute path names in 
the archive
Prepare volume #2 for /dev/fd0H1440 and hit return:
#
Note that you should format the floppies before you begin the backup, or else use another window or virtual terminal and do it when tar asks for a new floppy.
After you've made a backup, you should check that it is OK, using the --compare (-d) option:
# tar --compare --verbose -f 
/dev/ftape
usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
....
#
Failing to check a backup means that you will not notice that your backups aren't working until after you've lost the original data.
An incremental backup can be done with tar using the --newer (-N) option:
# tar --create --newer '8 Sep 1995' 
--file /dev/ftape /usr/src 
--verbose
tar: Removing leading / from absolute path names in 
the archive
usr/src/
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/modules/
usr/src/linux-1.2.10-includes/include/asm-generic/
usr/src/linux-1.2.10-includes/include/asm-i386/
usr/src/linux-1.2.10-includes/include/asm-mips/
usr/src/linux-1.2.10-includes/include/asm-alpha/
usr/src/linux-1.2.10-includes/include/asm-m68k/
usr/src/linux-1.2.10-includes/include/asm-sparc/
usr/src/patch-1.2.11.gz
#
Unfortunately, tar can't notice when a file's inode information has changed, for example, that its permission bits have been changed, or when its name has been changed. This can be worked around using find and comparing current filesystem state with lists of files that have been previously backed up. Scripts and programs for doing this can be found on Linux ftp sites.

12.4.2. Restoring files with tar

The --extract (-x) option for tar extracts files:
# tar --extract --same-permissions 
--verbose --file 
/dev/fd0H1440
usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
usr/src/linux-1.2.10-includes/include/linux/kernel.h
...
#
You also extract only specific files or directories (which includes all their files and subdirectories) by naming on the command line:
# tar xpvf /dev/fd0H1440 
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
#
Use the --list (-t) option, if you just want to see what files are on a backup volume:
# tar --list --file 
/dev/fd0H1440
usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
usr/src/linux-1.2.10-includes/include/linux/kernel.h
...
#
Note that tar always reads the backup volume sequentially, so for large volumes it is rather slow. It is not possible, however, to use random access database techniques when using a tape drive or some other sequential medium.

tar doesn't handle deleted files properly. If you need to restore a filesystem from a full and an incremental backup, and you have deleted a file between the two backups, it will exist again after you have done the restore. This can be a big problem, if the file has sensitive data that should no longer be available.

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