Most has been said, but I think responders failed to make the link to typical Linux virtual machines. In a way, a large Linux machine will beave like the 2nd worst case scenario that Jim describes. Apart from the initial load that Barton outlined, a large Linux virtual machine will continuously show this behaviour.
For those who care, let me add some detail to the vague description. Consider a Linux machine that runs a process frequently that needs a little bit of CPU time and some memory to do things, say 10 MB. When Linux has some 100 MB available for that, it will roughly allocate those 10 MB in different corners of that 100 MB every time, so after 10 times it has touch all 100 MB. When the process runs frequently enough (or when something else keeps that virtual machine in queue) this will show to z/VM a working set of 100 MB. If the Linux virtual machine is smaller and has only 15 MB free, Linux will be forced to re-use those same pages over and over again. Thus the working set on z/VM is 15 MB rather than 100 MB. Similar things apply to swapping to VDISK. When a page is swapped back in, it will normally not be read again. The allocation algorithms in Linux make it use fresh pages for each new swap out. So when Linux has 1 MB swapped out and is swapping a little bit now and then, over time it will use your entire VDISK and force z/VM to come up with all the paging space to carry that VDISK. If you allocate the same amount of VDISK in a few chunks and swap with different priority, your force Linux to re-use the same pages. The unused VDISK does not take resources on z/VM anymore, and the reference rate to the used portion is so high that it is less likely to be paged out by z/VM. Rob
