VMware RHEL5 guest OS memory cache release

dap263 · 07-13-2010, 12:58 PM

From everything I have read, RHEL will use all available/unused RAM for caching to increase performance.

However, this becomes an issue for me in a VMware environment. VMware (ESX4) looks for guest OS to let it (ESX) know when the guest is done using the memory so it will make it available it to another server.

So lets say I have a guest that has a dB, the dB runs an intensive job, it finishes and now its idle. But since it cached all the memory, VMware still thinks that the dB server needs it, so it doesnt release the RAM to be used by my webserver that needs the extra RAM. Sure I can live with as many guests as I have RAM, but that is a waste of a huge virtualization feature, since over all my CPU utilization is under 10% and my RAM utilization (as far as VMware knows) is 75%.

Any suggestions on how to tell ESX or RHEL to release the memory?

AlucardZero · 07-13-2010, 04:42 PM

Have ESX set hard limits on the VMs?

You could tell RHEL to drop its disk caches with

Code:

echo 3 > /proc/sys/vm/drop_caches

dap263 · 07-14-2010, 08:41 AM

Quote:

Originally Posted by AlucardZero

Have ESX set hard limits on the VMs?

You could tell RHEL to drop its disk caches with

Code:

echo 3 > /proc/sys/vm/drop_caches

That serves the purpose for a single use.

Should I cron that? However, would that just kill the performance on the server?

I was hoping there was a setting/tweak that could be done to limit the time cache was kept.