On 9/12/06, Austin, Alyce (CIV) <[EMAIL PROTECTED]> wrote:
In regards to paging, should MDC be off for a Linux guest running under z/VM? That is, MINIOPT NODMC to reduce paging?
There's no universal wisdom on that. Most of us agree that it is not useful to use MDC on Linux swap disks. But swapping is done better to VDISK anyway, so that is not really an issue. When there is no I/O to the swap disk, NOMDC or not is a moot point as well (and you would not bother selecting the disks for MOMDC). If you need to prevent MDC using too much of your real memory, you must tune the arbiter. Whether MDC is good for other Linux disks is the question. Ideally you would rather have Linux to cache its data than have z/VM do MDC (especially since MDC is not as effective). And Linux does that anyway if you give it the memory. But Linux will continue to cache its data even when it has little work to do and thus prevent the other Linux servers (that may have work to do) from keeping their cache. If you'd push hard enough, the Linux cache will be paged by z/VM and the result of that is normally unpleasant. The idea is that you keep the Linux servers relatively small. That way they're not too much trouble for z/VM and suffer less from z/VM paging. Obviously the small Linux servers have less room to cache stuff and will be doing more I/O when they have work to do. The small Linux servers now leave more room for z/VM MDC and it's the Linux servers with I/O (the ones that have work to do) that take advantage of that. This way you share the white space among the virtual servers. There is a cost involved with sharing, like with any sharing. Whether this works for you depends among others on the utilization of your servers (percentage of time they have work to do). Rob -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/