On Fri, Apr 01, 2005 at 08:20:18PM +0100, gj wrote:
> Hi
> 
> I'm trying to set up a backup system for 7 networked machines. Each
> machine has an average capacity of about 200GB (not all capacity used
> on machines), but due to resource limitations, we're backing up onto
> about 480GB of space on the hard drives of our backup server.
> 
> I've split the 480GB into 4 virtual tapes of 115GB each (so it
> definitely is less than the actual space we have):
> 

I'd do a calculation something like this.

  # sys X capacity/sys X ave fraction filled X ave compression ratio

Might be something like:
  7 system X 200GB/system X 0.4 X 0.5  ==  280GB

That would be the approximate size of all the data to be backed up,
after compression.

Next, how often do you expect to run amanda between level 0 (full dumps).
This is amanda's "runspercycle" parameter.  Say you are running amanda
daily and you want a level 0 each week, there would be 7 runs/cycle.

Remembering that amanda spreads its level 0's over all the runs gives us
280GB / 7 runs or an average of 40Gb of level 0 dumps per day.  Adding in
a bit for daily incrementals and some for daily variability, maybe 60GB
per tape would be needed.  That would seem to be 8 tapes of 60Gb == 480GB.

If the 480GB were all from on pool/file system, I'd be tempted to suggest
you allocate tapes beyond the apparent capacity of the file system.  If
you create an 80GB vtape, it does not take 80GB of the 480.  It takes
whatever is written to that tape during an amanda run.  So if you create
8x80GB, or 10x70, as most tapes will only have 45-60GB of data written to
them, it may never exceed the 480GB.  If it does, amanda will react like
it would to a broken physical tape.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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