2010/2/4 van Sleeuwen, Berry <berry.vansleeu...@atosorigin.com>: > We have some oracle guests that have been set to 1G. And they actually > need only 300M. The rest is occupied in cache (between 650 and 710M).
Be aware! The Oracle SGA lives in Linux page cache, together with in-use programs and shared libraries. So on Oracle systems it is normal to see a large page cache and it is not all waste. The Oracle instrumentation can tell you how much of the allowed SGA size is actually being use (and whether that is enough). When the system sees enough data, it will eventually grow to use all allowed SGA space. Without access to those metrics, you could write a "1" into the /proc/sys/vm/drop_caches to have Linux drop all other stuff from page cache. You will see Linux reclaim some of that in a few minutes, and that might be a good stake in the ground. > I'd like to decrease the machines but it is quite hard to convince the > oracle group that the machines do not need the memory. Instead, they > even want to increase to 2G because the books tell them to. While the > documents suggest the oracle will benefit from the increased memory, in > reality VM will page more and the effect is an overall decrease in > performance for all guests. You're correct that there is a cost for increasing the virtual machine size, but there is also a cost for not increasing it. You need to measure and analyze that. And you have the product that that can help you understand the cost rather than tune by fear and doubt :-) There's always a trade-off and your analysis should include the utilization of the virtual machine. When the server is rarely active, it may be acceptable when it is less efficient since that reduces the resource cost when it is idle. But servers that are very busy will benefit from efficient operation and you're less concerned about the idle footprint. > I would suggest to keep the guest as small as possible. Also try to > limit SGA and such. Among the "such" is also the PGA as sized by the DBA. That does not live in page cache but in anonymous memory in Linux (the "used" part that is not cache). Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
