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Using tar to Perform Incremental Dumps

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Performing incremental dumps is similar to performing full dumps, although a few more options will usually be needed.

You will need to use the `-N date' option to tell tar to only store files that have been modified since date. date should be the date and time of the last full/incremental dump.

A standard scheme is to do a monthly (full) dump once a month, a weekly dump once a week of everything since the last monthly and a daily every day of everything since the last (weekly or monthly) dump.

Here is a copy of the script used to dump the filesystems of the machines here at the Free Software Foundation. This script is run via cron late at night when people are least likely to be using the machines. This script dumps several filesystems from several machines at once (via NFS). The operator is responsible for ensuring that all the machines will be up at the time the dump happens. If a machine is not running, its files will not be dumped, and the next day's incremental dump will not store files that would have gone onto that dump.

# Dump thingie
set now = `date`
set then = `cat date.nfs.dump`
/u/hack/bin/tar -c -G -v\
 -f /dev/rtu20\
 -b 126\
 -N "$then"\
 -V "Dump from $then to $now"\
echo $now > date.nfs.dump
mt -f /dev/rtu20 rew

Output from this script is stored in a file, for the operator to read later.

This script uses the file `date.nfs.dump' to store the date/time of the last dump.

Since this is a streaming tape drive, no attempt to verify the archive is done. This is also why the high blocking factor (126) is used. The tape drive must also be rewound by the mt command after the dump is made.

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