The answer partly depends on whether (or not) you have PAV support in your DASD hardware. Last I knew, only dynamic PAVs will provide performance if you have multiple page datasets on a single volume (UCB); statis PAVs will not, and hyper-PAV support is still on the drawing board. IBM has recommended a minimum of 4 locals (or 4 paging paths).
Now, for most environments, paging is not much of an issue today ... storage is plentiful and cheap. Issues can arise when an SVC dump occurs. The SVC dump is first written to a dataspace. This may cause frames from other applications to get paged out. (The system will probably go disabled for some time around here ... not exactly sure when. You need a frozen picture of the system to be included with the dump.) Once the dataspace is populated, the system will being processing again, and the dataspace is written to the dump datasets. Storage will then come available for the frames that were paged out to make room for the dataspace. (How much room - it depends ... think of a large CICS region using a multi-gigabyte DB2 region ... you get the idea.) You really want this to happen quickly and non-disruptively; this means many page datasets and many ways to get to them. So, unless you have dynamic PAVs, lots of locals is still better. DS/8000s will let you carve up the DASD however you see fit. (I like 3390-2s for locals ... YMMV. I have hyper-PAVs and z/OS 1.9. I don't think that the dynamic PAV support is yet available for hyper-PAVs ... IBM?) ---------------------------------------------------------------------- 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
