If you are talking about regular incremental backups, yes I think TSM uses
more tape than most other backup products.
The reason: most other products are doing at least periodic full dumps.
You write a large amount of data to a tape, then the tape is expired after
xxxxx days and the dumps are done again and the tapes get rewritten and are
full..
With TSM and the "perpetual incremental" plan, you send only a small amount
of data over the network becuase you never have to resend unchanged data.
But that means each client adds to its data storage a little at a time, and
as file versions expire, you get "dead" space in the tapes. In our
environment, I find it is difficult to keep tapes more than 65% full, on
average, even with very aggressive reclaiming. Plus you need additional
tapes for things like DB backups, that are never full; copy pool tapes that
don't get filled before they are sent offsite, etc..
So here is a formula I use as a starting point:
Take the # of GB of data on the client.
Add 5% for the number of versions you keep. For example, if you allow 6
versions, then multiply by 1+(.05*6) = 1.3. (This assumes 5% of your data
changes daily).
Divide by 2.5 (assuming a compression ratio of 2.5; in some environments you
get more, sometimes less)
Multiply by 2 if you are creating a copy pool.
Divide by .65, as your tapes will be at BEST 65% full.
Divide by the RAW capacity of your tape:
So for a client with 50 GB of data, keeping 6 versions:
50 GB
*1.3 = 65GB
/ 2.5 = 26 GB
*2 = 52 GB
/ .65=80 GB
SO if you are using 20 GB tape (raw capacity).
80GB / 20GB = 4 tapes
Now you have to adjust ALL those values based on your own
installation/experience; if you have huge clients that are data collectors
so that LOTs of the data changes daily, use a number larger than .05 for
data changed. If your clients are huge web severs with a high percentage of
.tif and .gif files that don't compress will, use a compression number
smaller than 2.5. If you have few tape drives and it's difficult to find a
time window for reclaims, use a number smaller than .65 for your average
tape util....
as I said, this is a starting point!
Hope that helps...
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