NSTM [1]... On Thu, 30 Oct 2003, Post, Mark K wrote:
> My point was that since Red Hat requires 256MB to be supported, you need to > be able to reproduce any sort of problem while running in that > configuration. You don't question the 256MB requirement? To me, this is troubling. Red Hat knows (or should know) that one of the objectives when running Linux under z/VM is to *reduce* the amount of storage configured to guests. If this is a hard requirement, then Red Hat has just reduced the number of guests that can be supported in a given z/VM system by at least a factor of two (did they require 128MB before? A certain site I'm associated with runs most of theirs with 64MB, so for us it's a factor of four). Given the numbers that Mike presented, why should 256MB be required? It obviously does not need it... > If the hypothetical performance problem goes away, then there's no problem > to report, is there? What if a problem exists in the virtual memory process? Sure the problem will go away if you don't swap (or don't load the swap process sufficiently to exhibit the problem)... I agree with Rob: how can anyone diagnose a problem if you have to change the machine configuration to get support? Again, if this is Red Hat's requirement it smacks of the usual discrete-server mentality of solving performance problems by throwing hardware at them. And, a lack of understanding of the platform: the use of properly-configured VDISK swap under VM has been shown to provide a very high-performance swap device, so swap is not necessarily evil. > Whatever you're doing actually requires that amount. Maybe, but does it all have to be in storage, without swapping? Isn't that the reason for swap: to increase the amount of virtual memory available to the kernel, allowing the kernel to overcommit the physical resource of the machine? As long as the working set fits within the total virtual memory available (real + swap space), we should be okay. I might be reading this the wrong way, but there is a difference between saying (for example) "your working set is 200MB, so because your storage size is 128MB you will experience performance degradation due to swap", and "your working set is 200MB, we told you to have 256MB storage, go away". What about reconfiguring the machine keeping the total virtual memory amount the same? For example, going from 128MB storage / 128MB swap to 256MB storage / no swap. If the problem still exists, is this because of the same software problem or because the machine now has no swap space? ;-) I can see a lot of hard work done by folks like Rob being thrown out the window thanks to an ill-considered number in a document. Customer has a Redbook in one hand and a support contract in the other, I think I know which way they will go... Hoo-roo, Vic [1] Not Shooting The Messenger ;-)
