On 9/29/06, James Melin <[EMAIL PROTECTED]> wrote:
The reason I ask is that we're running overcommited here and our head z/VM person had me stop using vdisk a long time ago because he was worried about our memory allocation. 6 gig real 1 gig expanded here, and we're using a goodly amount of it.
Some of the fear for using vdisk comes from the days where CMS users used it as temporary disk space. That's a bit different from what we do with Linux swap disks. And some of us had the opinion that vdisk would not be paged out. It is good to overcommit a resource because that's the only way to share it. You just should not overcommit all your resources at the same time. I can't tell whether you should or should not use vdisk without seeing your data. I'm not even sure how you define your degree of memory overcommit and what data you use to measure it. But I can experiment and measure 100 running servers on a P/390 (128 MB real memory) and show what the performance penalty is for that. With z/VM 5.1 and before it was easy because Linux swapping to disk was a *real* bad idea for most installations. With z/VM 5.2 you can get away with a lot more. When you define big "just in case" swap disks it's almost for free as long as they don't get used. And when they do get used by some out-of-control process, it will just slow down things a bit if you tuned the system correctly. I only recall one case where that cause problems, and that was a system where they failed to use expanded storage for paging. If you don't trust it, do your experiments and measure. -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
