george (11/11/82)
the suggested way to perform dumps given by Sys III is quite different than Berkeley's recommendations. Sys III recommends that the Tower of Hanoi sequence refers to tapes while Berkeley says it refers to dump levels. Furthermore Sys III implies that one is never to use dump levels 2 to 8. does anybody understand what the tower of hanoi sequence has to do with dumps or why Berkeley's and Sys III 's recommendations are so different .
shp (11/12/82)
The Tower of Hanoi is used to keep the number of tapes needed for a complete restore to an abosolute minimum. You have to really understand the concept of dump levels. From dump(8): 0-9 This number is the 'dump level'. All files modified since the last date stored in the file /etc/dumpdates for the same filesystem at *lesser* levels will be dumped. If no ... Consider the following dump cycle: day 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 level 0 3 2 5 4 7 6 9 8 9 9 1 3 2 5 4 7 6 9 8 9 9 0 On day 01 a zero (complete) dump is performed. On 02, a level 3 dumps all files modified since day 01. The level 2 dumps all file modified since day 01. If you had to restore now you'd only need the tape sets from day 01 and 03. On day 04 you do a level 5 dump. This dump only dumps files modified since day 03. And so on ... After dump on day 12, the system can be restored using the 0 and 1 level tape sets. The Tower of Hanoi minimizes: - number of tape sets needed to fully recover - the average number of files dumped daily - sam praul ...decvax!ittvax!shp
gwyn@Brl@sri-unix (11/21/82)
From: Doug Gwyn <gwyn@Brl>
Date: 17 Nov 82 11:31:39-EST (Wed)
The idea of the System III dump recommendation is to minimize the number
of dump tapes while permitting recovery from an unreadable "daily"
backup. In fact dump levels 2..8 are intended to be used but only
after the scheme has been running for some time.
Basically,
LEVEL 0
LEVEL 9 (Hanoi)
LEVEL 9
LEVEL 9
LEVEL 9 nearly full
LEVEL 1
LEVEL 9 nearly empty
LEVEL 9
LEVEL 9
LEVEL 9 nearly full
LEVEL 2
LEVEL 9 nearly empty
...
>From my experience with tape dumps I would say that this is a nice
scheme, but its bookkeeping is trickier than the daily/weekly/monthly/...
scheme even though it uses fewer tapes.
I would add the recommendation that one always have TWO copies of
a "baseline" dump (such as levels 0, 1, 2... above) since Murphy's law
seems to apply to magtapes with a vengeance.