This question is not well understood by most and the rules have changed
drastically. Since we have installations with 30gb or even 50GB of
expanded storage being used very effectively, i know there are some
ROT's out there that are slightly "bogus".
IF your linux servers are not polling (or i hear ibm now calls polling
servers "noisy servers"), rules are different. But if you want to run
any of the apps that poll (Websphere, DB2, SAP) then your servers are
not going to drop from queue and there are different requirements. If
you only run Oracle which does not poll, then less expanded is ok.
Take a very busy server, and an idle "noisy / polling" server. neither
drop from queue, cp must steal pages from both pretty much equally.
So in one case, CP will be stealing the wrong page from the busy server,
and this pattern is easily shown.
The question then is how big a buffer is needed to buffer the pages you
really do NOT want written to disk? as big as possible. given that cp
WILL be stealing pages, expanded storage provides a buffer to ensure the
page is really the correct page to write to disk.
There are two items to check, if you are stealing pages directly to
disk, then you don't have enough expanded storage. if you have barely
enough expanded, then your page blocksize goes down. the more expanded
storage, the bigger the buffer, the bigger the page blocksize, the lower
the page i/o rate.
my vote would be with only the choices you gave, 25/10.... I would
rather steal more pages and let them sit in a very fast buffer than
steal less pages and have good pages go to disk.
Martha McConaghy wrote:
I know that this question has come up numerous times in the past, but
I'm don't recall ever seeing a consensus on it. I know that z/VM needs
expanded storage. However, deciding how much is the trick.
We will be moving to a z10 early in the new year. I'm trying to decide
how to divide the storage. If an LPAR is going to be running a lot of
Linux guests (some of them quite large), is it better to put as much storage
into main as possible with a small expanded....or cut back on the amount of
main and put it towards expanded? I've done the former in the past. However,
we now have a lot more Linux servers than ever before, some of which are
running pretty heavy apps, like Oracle. The current LPAR does do a lot of
paging to expanded and disk. While there will be more storage on the z10, it
isn't a lot amount more.
So, for example, if I have 35G to use for 1 LPAR, would it be better to
define 30G main and 5G expanded or 25G main and 10G expanded?
Martha
