--On Friday, October 03, 2003 16:00:16 -0700 "S. Keel" <sakeel AT u.washington 
DOT edu> wrote:

> I'd like to bouce this off you all to make sure that I'm understanding
> things correctly.
> 
> Lets say that my total data to be backed up is 83.5GB, and from some
> initial trial runs, I've achieved at least 50% compression, bringing that
> down to 41.75GB.
> 
> I've set my dumpcycle to 1 week, and runs per cycle to 5 so that the
> backup runs only on weekdays (when I'm available to deal with possible
> errors, etc.).
> 
> Since my tapes are 20/40GB, given the compression that I'm getting, I
> should only need 3 tapes for the entire dump cycle, correct?  2 tapes to
> handle the dump cycle, and one extra for security.

Since each day's run of Amanda wants it's own tape. you would need
at least 5 tapes.  10 would be a more desirable number so you could
still recover older data if you had a bad tape, and having a couple
of extras would be helpful.

> If I'm understanding how Amanda works, during the course of the dump
> cycle, she performs a full backup of each disk in the disklist.  She uses
> her keen wisdom in determining on which days she does a full backup of a
> particular disk, and on which days she does an incremental; nonetheless,
> by the end of the dump cycle, she will have consumed a total of 41.75GB of
> tape.

It depends on how big your disks are and how much the data changes each
day.  If your data is mostly static you would get the full-sized level 0
once per dumpcycle and small incrementals the other days.  If it changes
a lot (like a mail or news spool) your incrementals might be almost as
large as the fulls.  If you have a disk larger than a tape it will never
get backed up since it will never be able to do a full.
   Best case, if you have lots of small disks and your data never changes,
would be your 47.75mb / 5 runs = 10mb per day once things level out.  If
your disks don't compress evenly it will take a little longer to get
leveled out, and if you have a large one that doesn't compress much at
all it may not fit on your 20mb tape.
   You might also want to check the archives for the previous discussions
of software vs hardware compression.

Frank

> 
> Does this all sound right to you?
> 
> Thanks,
> Stefan



-- 
Frank Smith                                      fsmith AT hoovers DOT com
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