[Gllug] Unix dumps and the tower of Hanoi

David Damerell damerell at chiark.greenend.org.uk
Fri Apr 8 13:59:08 UTC 2011

On Friday, 1 Apr 2011, James Hawtin wrote:
>I have tried to understand this thing however I just don't get it. Why do
>Unix people like to use this number sequence for backups?
>3, 2, 5, 4, 7, 6, 9,

The intention is to minimise the number of tapes that will be needed
if you have to do a restore - each even-numbered tape renders the
previous odd-numbered tape obsolete.

>The above is functionally the same as:-
>2, 2, 3, 3, 4, 4, 5,

This is true. I think the motivation for the sequence above is that
the chance of restoring the wrong incremental is reduced, because each
one has a unique incremental number.  In your scheme, a given tape
might have one or two "level 2" dumps on. The potential for annoyance
is greater. I am just speculating here.

>one. However it does not say this in the any manual most people don't use
>tapes these days anyway... is it just a throw back to the old days of tape 

Certainly the whole business with incrementals largely reflects the
idea that you are using expensive tapes and a full backup can span >1 tape.

David Damerell <damerell at chiark.greenend.org.uk>
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