ADSM-L

Re: Formula to calculate # tapes required

2001-02-15 18:34:52
Subject: Re: Formula to calculate # tapes required
From: Paul Zarnowski <vkm AT CORNELLC.CIT.CORNELL DOT EDU>
Date: Thu, 15 Feb 2001 18:36:34 -0500
At 04:50 PM 2/15/2001 -0500, Prather, Wanda wrote:
The formula I use is:

Need to back up xxx TB
Divid by the number of GB that will fit on a tape.
Adjust as you have suggested for extra versions
Divide by .65 on the assumption that the not-yet-reclaimed tapes will be, on
average, 65% full
Multiply by 2 for the offsite copies

This is pretty close to what I use, too, but make sure you factor in
compression somewhere.  Also, if you are using collocation, then you also
need to consider how rapidly your data is changing.  If you have high data
turnover, then you will be reclaiming data more frequently.  If you don't
have enough tape drives or wall clock time to handle this, you will need
more tapes, plan on a lower average tape utilization, and raise your
reclaim threshold to relieve the pressure on reclaim.

This reclaim issue will not surface at first in a new robot.  You will only
run into it over time, after a bunch of your tapes have filled up and you
start doing more reclaims.  This issue is also dependent on the tape
technology you are using, as some perform reclaims faster than others.

..Paul