Tom Duerbusch wrote:
VDISK is different. Actually, it isn't vdisk is different, it is memory
is different.
CP lets me throttle every other shared resource.
I can SET SHARE to throttle or cap CPU.
I can throttle or cap I/O rates for a particular user.
But once I give a user access to virtual storage (max guest machine
size plus vdisk(s)), I can't throttle his usage. And in the very vast
amount of cases, I wouldn't want to. But if/when that time comes, when
vdisk starts getting pounded, in effect the working storage size of that
guest, jumps to something you never seen before, or expected, and even
when planned for, impacts the production systems (as they start getting
paged out).
I'm wondering how much virtual storage you're handing out; I recall that
in my early days on this list, folk were bandying about (almost)
unbelievable (to me) small numbers. 64 Mbytes and less in some cases, I
think.
There were also assertions that vendors don't know what they're doing
anf their numbers should be taken with salt.
What they need to run on IA32 hardware (especially under Windows) isn't
a good guide to what you need to give them in a virtual environment.
If you're handing out 512 Mbytes eash (randomly-chosen number), your
system may actually work better if you hand out 128 Mbytes + 128 Mbytes
VDISKplus 512Mb swap on DASD. Or different numbers, you can't be sure
without the right tools.
Tuning MVS 101: First, measure the performance.
btw The rate of paging is fairly unimportant: in the 80s, we had an
Amdahl 4360 paging at 600 pages/sec. We looked at the numbers, we looked
at each other, we shrugged our shoulders: "TSO seems okay."
Responsiveness and throughput are important. In our case, paging was
"fairly brisk" but overall balance seemed good (CPU was suitably busy too).
--
Cheers
John
-- spambait
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