Others have spoken on this and they've spoken well... I've seen a situation such as yours, albeit with slightly smaller numbers. The 8GB virtual requirement turns into 8GB real or (more probably) 8GB of mixture of real and aux.
There's a trade-off here (and I think it has the opportunity to be a fairly unpleasant one): Too small a value and you get partial dumps (with the potential for partial fixes). :-) Too large and you have real memory and paging space consumption issues. For what it's worth, and this is part of the reason for replying, I think we need to develop a better understanding of how dumping works - from the address space perspective... Some products are pretty smart at dumping (though maybe could be smarter) deploying techniques such as sequencing dumping in an appropriate way and avoiding dumping the whole of large (largely fruitless) areas. There are probably other tricks but I'm not close enough to it to know. So, a question: Would it be helpful to have documented a snapshot of how eg DB2, CICS, WAS, WMQ, IMS etc dump? I've thought so for some time and would be quite willing to attempt to collate some of this - with the help of relevant development groups. In a way I've already started: My "Memory Matters" presentation now has some stuff on dumping and configuring for it. I suspect most of my audience wonder why. It's PRECISELY for situations like yours that I think the material worthwhile. So, for example, suppose we only got the first 4GB of a 10GB DB2 DBM1 dumped before the space ran out. What's the impact? Similarly, suppose 10 CICS regions dump in parallel and we run out part way through? Etc etc. And back to the original question: Some installations take a "dump must be ultra-fast event" stance and others take a "we can't afford huge amounts of idle memory" stance. I think installations need to take SOME stance, though it's probably going to be in between these two. Thoughts? Thanks, Martin Martin Packer Performance Consultant IBM United Kingdom Ltd +44-20-8832-5167 +44-7802-245-584 [email protected] "They're figuring out that collaboration isn't a productivity hit, it makes them smarter." Sam Palmisano on BlogCentral, 26 November 2008 From: Lizette Koehler <[email protected]> To: [email protected] Date: 07/04/2009 20:11 Subject: MaxSpace and Real Memory Sent by: IBM Mainframe Discussion List <[email protected]> I am currently working with our DB2 group on a problem that requires an SVC dump. IBM is asking for us to set maxspace to 8g. I only have 8g real memory on this lpar. We currently have maxspace set to 2.5G. What would be my impact if I set the maxspace to 8G on an LPAR with only 8G real memory. Will that cause me any problems or overhead? How can I calculate what I will really need for this DB2 system to dump? Lizette Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
