ADSM-L

Re: IBm Caching recommendations

1997-04-18 20:06:24
Subject: Re: IBm Caching recommendations
From: Andrew Raibeck <storman AT US.IBM DOT COM>
Date: Fri, 18 Apr 1997 20:06:24 -0400
Classification:
Prologue:
Epilogue:

Hi Jerry,

I'm not aware of any "IBM official party line" on this issue, but I'll share my
thoughts.

Besides extra database space, another potential "gotcha" with caching is the
time it takes for ADSM to delete the cached bitfiles when it needs to use that
space in the disk storage pool. Since we have a pointer in the database to that
cached copy, that pointer must be deleted from the ADSM database before the
space can be reused. This is going to incur some overhead. The amount of
overhead will depend on how many files must be deleted in order to free up the
necessary space. A while back, ADSM server development made some improvements
in the algorithms that do the cached bitfile deletion... but the overhead is
still there. I have no numbers on the significance of this overhead; in most
cases, it's probably negligible. But if you are having problems meeting your
backup window, this is something to consider.

When deciding whether to cache, some things to consider might include (this is
not an exhaustive list):

How long does the data stay on disk? i.e. do I back up to disk, and migrate
the pool during the day? Or does the data stay on disk for a while before
migrating (maybe I've got a lot of DASD)?

How many restores do I do every day?

With caching turned on, how many of those restores come from disk?

How much does a tape mount cost me?

How long does it take to mount a tape?

Do I have an automated tape library?

Am I meeting my backup window?

Just my $0.02,

Andy Raibeck
ADSM Level 2 Support
---------------------- Forwarded by Andrew Raibeck/San Jose/IBM on 04-18-97
04:15 PM ---------------------------
04:15 PM ---------------------------

        owner-adsm-l @ VM.MARIST.EDU
        04-18-97 04:06 PM
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To: ADSM-L @ VM.MARIST.EDU@internet
cc:
Subject: IBm Caching recommendations

Date:     April 18, 1997           Time:    2:27 PM
From:     Jerry Lawson
          The Hartford Insurance Group
          (860) 547-2960           jlawson AT thehartford DOT com
Attention IBM ------

We haven't had a good discussion on caching lately...

It was recently pointed out to me that, at least for the MVS server, the
recommendation from you (IBM) is to not use caching.  Can you explain the
shift (if there was one) to this position.

I think we are all aware of the issues of having duplicate DB entries for the
cached files, but does it really add that much?  My customers seem to like
the benefits of having quick recall on files that may already be on tape.
What are the overhead and liability issues with caching enabled?


                                         Jerry
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