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<H1><A NAME="SECTION001140000000000000000">Simple backups</A></H1>
<P>
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 <b>full backup</b>, the subsequent
ones are <b>incremental backups</b>. A full backup is often
more laborius 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 optimized 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.
<P>
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 2-5 again. You don't want to overwrite tape 1 with
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.
<P>
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.
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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