Dump/Restore help needed

[zemon at felix.UUCP]

Larry W. Virden (lwv at n8emr.UUCP) writes:
> Hi there!  I have seen the 3-4 paragraphs or so in the supplimental or is
> that the administrative manual which tells me that if I do dumps using 
> levels 3 2 5 4 ... I get a 'tower of hanoi' style dump.  Unfortunately,
> I am not sure a) what the level numbers mean, and b) in what order the
> restores are supposed to be done.  Since someone out there connected with


The oblique reference to towers of Hanoi is one of the more
rediculous examples of documentation that I have ever seen.
It has been in the Unix manuals for years and apparently no
one at DEC's technical writing department understands it
either or they would have replaced it!  :-)  So here is my
long explanation.  Hope you find it helpful.  Feel free to
ask more questions.


Start by assuming two goals for your backup procedure.  1)
Dumps, which are performed daily (i.e., very frequently)
should not take too long.  2) Restorations, which are
performed infrequently, can take significantly longer.  On
the other hand, if you can still achieve #1 and avoid
reading *all* of your backup tapes to do a restoration then
you can save some time on those infrequent occasions when
you do need to restore.

You can achieve #1 by only backing up the files that have
changed in the last day.  The dump levels make this easy
since they mean "dump all files which have changed since the
previous dump with a LOWER dump level."  In other words, if
you type "dump 3 ..." then the dump program will look in
/etc/dumpdates to determine when the most recent dump of
level 0, 1, or 2 occured.  Only the files which changed
since the most recent of those three levels will be dumped
this time.  So you can dump one day's work at a time with
the following sequence:

    Su M  Tu W  Th F  Su M  Tu W  Th F  Su M
    0  1  2  3  4  5     6  7  8  9     0  1 ...

You back everything up on Sunday (dump level 0 means backup
everything).  On Monday evening you back up the work done on
Monday (and Sunday but who works on Sunday?  :-)  On Tuesday
evening, dump looks for the next lower level dump from level
2 and finds a level 1 dump done on Monday so it picks up
just the files changed on Tuesday.  And so on.

There are a couple of problems with this sequence, however.
One is that if the disk crashes on the second Friday you
must restore all ten of your tapes to get back to Thursday
night.  You would start with tape 0 and then read all the
others in increasing order.  Since restore is *much* slower
than dump, you will be slinging tapes for a *long* time.
The other problem is that work done on the second Friday is
not backed up until later in the weekend.  This gives a
wider window between when the work was done and the backup
is made and that means more time for gremlins to strike.

You can reach a compromise by being willing to back up two
days worth of work in an evening.  Consider the following
sequence, the infamous Towers of Hanoi:

    Su M  Tu W  Th F  Su M  Tu W  Th F  Su M
    0  2  1  4  3  6     5  8  7  9  9  0  2 ...

You start with a full dump on Sunday.  On Monday you dump
one day's worth of work.  On Tuesday, dump looks for a
previous dump with a level less than 1 and dumps all files
changed since Sunday (since the 0 on Sunday is less than 1
but the 2 on Monday is not).  On Wednesday, you dump all
changes made since Tuesday (1 < 4).  On Thursday, you again
dump all the changes since Tuesday (1 < 3).  And so on.

Now if you have to restore on the second Friday, you only
need to read half of your tapes.  In order, they would be:

    0 1 3 5 7 9

This sequence also gives you a bit of redundancy, although
it is not intended to provide complete redundancy.

Cheers,
	-- Art Zemon
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