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
