Re: [ADSM-L] How are tapes declared full

2007-04-19 11:23:41
Subject: Re: [ADSM-L] How are tapes declared full
From: Kelly Lipp <lipp AT STORSERVER DOT COM>
Date: Thu, 19 Apr 2007 09:23:37 -0600
It's full when TSM reaches the End of Tape (EOT) mark.  At that point,
it knows exactly how much data it put on the tape.

It shows a percentage utilized as less than 100% when data has been
expired on that tape.  By default, when a primary tape pool percent
utilized reaches 60% (the reclaim percentage) that tape becomes eligible
for reclamation.  At that time all the valid data on that tape will be
moved to other tapes and returned to scratch.

The way it determines estimated capacity is interested.  In the case of
LTO2, it will show an estimate capacity of 200,000MB (that's the native
uncompressed capacity of the tape) until it writes more data than
200,000MB.  From then on, it reports estimated capacity equal to the
amount of data actually on the tape.  It ahs to do this as there is no
way to know how much data can actually be written (when compression is
used).  The only time you really know how much data can be written (when
estimated capacity = actual capacity) is when the tape is full.  So it
is not unusual to see tapes with widely varying capacities.

Another interesting fact that you might notice is an estimated capacity
of the storage pool.  That's computed by taking an average of the full
tapes and multiplying by maxscratch.  Just a useless bit of trivia to
add to your brain.


Kelly J. Lipp
VP Manufacturing & CTO
STORServer, Inc.
485-B Elkton Drive
Colorado Springs, CO 80907
lipp AT storserver DOT com

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Angus Macdonald
Sent: Thursday, April 19, 2007 3:52 AM
Subject: [ADSM-L] How are tapes declared full

Doing some maintenance, I notice that most of my LTO2 storage pool tapes
are designated as full, although their % utilised is sometimes less that
60%. The estimated capacities also vary from 320GB to over 600GB. I'm
not actually having any problems but how does TSM decide the estimated
capacity of a tape and whether or not it is full?

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