The biggest mistake people make when sizing TSM is in just summing up
the total amount of data they will back up.
Not only do you need additional space in the library for your inactive
versions, you must allow for the fact that  your volumes WILL NOT BE
FULL.  Data is constantly expiring, and on a given day a significant
number of your volumes will be partially full, but below your
reclamation threshold.  

My rule(s) of thumb:

1.For file server backups, start with the amount of data you will back
up.  Add .05 (5 per cent) multiplied by the number of VERSIONS you will
keep.  (IBM recommends 10% as a starting point, but I find the rate of
change on most file servers to be far less than that).

2.For large data bases, (Oracle, Exchange, etc.)  start with the amount
of data you will back up.  Multiply by the number of versions you will
keep.

3.Add some additional tapes for your TSM DB backups and scratch pool.

4.Assume that you will get at most a compression ratio of 1.5.

5.Assume that your tapes will be, on average 60% full.

6.Now that you have a total, remember that most data centers experience
a rate of growth in data storage of 50-100% PER YEAR.  Whatever you buy
for TSM, it better be expandable.

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Lepre, James
Sent: Wednesday, December 22, 2004 9:46 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: Tape Questions


Hello All,

    I have a question is there a rule of thumb for how many tapes will
be used, when using TSM.  I heard that for every Terabyte that is backed
up, 1000 tapes could be used.  I was just wondering, if anybody knew
something different.  I keep 2 versions for 7 days

Thanks

James