I have gone over the "WebSphere MQ V7.0 Features and Enhancements", the "Migration Guide"....and this was not mentioned once. I did find reference to this in SupportPac MP1G...but it was after the fact and I had already converted to V7.01.
In version 7.01 of MQ for z/OS....IBM changed the way messages are stored on the pageset. If a message is under 2K....it is stored 1 message per page. This was huge for one of my applications.....as they kept filling the pageset, and overflowing to the dead letter queue. It took a little while to figure this out. Regardless of the design implications....this application would get data all day, and then process it at night. Well it didn't take long to fill the pageset up to the 4GB limit. These messages are 153 bytes each.......so we use a 4K page to store 153 bytes. We went from approximately 20 messages per page....to one.
The solution was to allocate a new "extended" VSAM dataset to be able to go over the 4GB limit. Once this was allocated and formatted......I then dynamically added the pageset to the queue manager:
-qmgr DEFINE PSID(9) BUFFPOOL(3) DSN('MQM.qmgr.PSID09') EXPAND(USER)
Then I created a new STORAGE CLASS called EXPANDED to use this pageset...and finally, I had the application process all of the data...stopped all incoming channels so the queue stayed empty, and then reassigned the queue to the new storage class.
Remember when you add a pageset dynamically, you will still need to change the PROC to include the new DD statement for the pageset, as well as to update the CSQINP1 input dataset to include the pageset to buffer pool entry:
DEFINE PSID( 09 ) BUFFPOOL( 3 )
as well as the input for CSQINP2, where you assign the storage class:
DEFINE STGCLASS( 'EXTENDED') +
PSID( 09 )
There is also another work around that is talked about in the MP1G Performance SupportPac...and it is having this statement in your CSQINP2 DD statement:
REC QMGR (TUNE MAXSHORTMSGS 0)
This will have the queue manager store messages in the pre v7.01 way....more than one on a page.
It seems to me that this should have been pointed out in the migration guide, as well as mentioned in the features and enhancements book. Having to search for information on this after the fact, was not really what I had in mind.