ADSM-L

Does anyone truly understand (or use) DBPAGESHADOW?

2004-06-14 08:40:43
Subject: Does anyone truly understand (or use) DBPAGESHADOW?
From: Scott McCambly <mccambly AT ALLSTREAM DOT NET>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Mon, 14 Jun 2004 08:39:44 -0400
Hello everyone,

Here is an issue that I have never felt comfortable with, and it is
being raised yet again in our environment as we consider how to squeeze
every bit of performance out of our poor overworked TSM server (5.1.6.2
on AIX 5.2.... I know, I know... we'll get to 5.2. some day hopefully).

There are multiple references to using the DBPAGESHADOW option with DB
MIRRORWRITE in PARALLEL instead of SEQUENTIAL.
Here is one from a TSM Flash:

5)  Consider changing server options for the duration of the upgrade to
optimize TSM Database performance:

     - Set MIRRORWRITE DB PARALLEL and DBPAGESHADOW YES in the server
options file

     - Use optimal database  buffer pool size

Here is one from a recent TSM support call:

The parallel mirroring for the data base can slow down a server from the
performance perspective so the performance team recommends that the
data base mirroring be parallel.....This can cause an exposure where a
partical
page write can occur on each mirror for the same page.  with the
pageshadowfile TSM will
write the data there first so if there is a crash and have partial
page write to the
data base it can fix that page

I can understand the disk space benefit from using this WITHOUT a DB
mirror, but how can there be a performance benefit?
I am assuming that the PAGESHADOW write is synchronous with the database
write (otherwise, how could it provide the stated protection), so that
gives us the choice of the following two scenarios assuming we do want
DB mirrors in TSM:
a) If I am in DB sequential write mode, I do one write of pages to
mirror A, then one write of pages to mirror B, sequentially.
b) If I am in DB parallel write mode with DB pageshadowfile enabled, I
do one write of the dirty pages to the pageshadowfile, then once that is
finished I do my parallel writes of the same data to the two DB
mirrors.  This is 3 writes in total (granted that the last two are in
parallel)!  How can this possibly be faster than simply doing sequential
DB mirror writes?

Or is this just a non-issue for all you TSMers out there because nobody
is using this feature :-).

Thanks, and have a good weekend.

Scott McCambly

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