Known scalability issues as size increases above 256GB primarily involve z/VM's page replacement algorithms including the "Reorder" and "Demand Scan" functions; known symptoms include things like excessive system CPU overhead decreasing overall throughput, and temporary guest hangs as CP tasks process resources belonging to guests with very large numbers of pages resident. Other symptoms may be possible as well, depending on workload characteristics; we haven't experimented with, measured, and analyzed all that many different kinds of workloads in such large environments.
Regarding running right at 256GB, that is a supported environment, so should any issues occur, contact us via the usual support channels and we'll work with you to resolve them. Some of the same scalability issues can indeed manifest to some extent at sizes below 256GB, depending on workload, but generally to a degree that is more manageable and can be "tuned around". We understand the need to increase the supported storage size above the current 256GB limit. - Bill Holder, z/VM Development, IBM Endicott On Mon, 11 Oct 2010 17:00:36 -0400, Martin, Terry R. (CMS/CTR) (CTR) <[email protected]> wrote: >So the question is if for some reason you go over that 256G say to 265G what issues will you see, and how will the manifest themselves? > >And if I stay right at 256G will I see any issues being that it is right at the max? > > > >Thank You, > >Terry Martin >Lockheed Martin - Citic >z/OS and z/VM Performance Tuning and Operating Systems Support >Office - 443 348-2102 >Cell - 443 632-4191 > >
