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whit87654321
Contributor
Contributor

Cluster Memory Performance Chart question

On my VirtualCenter, when I look at the Performance tab for the one cluster I have of 7 hosts, the graph of Memory utilization shows up as consistently between 16 and 20 percent, even looking back a full month. I was curious why this is so stable at that level and why it never increases above 20 percent. It seems like a lot of capacity is not being used. I'm new to VMware and not fully up on the details of how ESX handles memory, so any information anyone has would be much appreciated.

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dnetz
Hot Shot
Hot Shot

Can you compare this to each host's memory load and see if it differs? Perhaps the hosts aren't loaded more than on average 16-20%? There is a lot of memory management going on inside of ESX, for example transparent page sharing reduces memory usage where memory pages are identical, so if you are running a lot of the same OS's and applications in your VM's, the memory load will decrease.

This document is a really good read for understanding how ESX handles memory: http://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdf

Also please note that there's a bug in vCenter 4 (with no available fix from what I know) that will affect the memory average percent statistic for the cluster, mine is always showing below 1% because of this. Bug number is 479003.

Hope it helps!

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weinstein5
Immortal
Immortal

Those metrics could be correct - ESX/ESXi have the ability to economize on memory by storing identical memory pages a single time - so if you have 20 VMs all doing the same or similar functions memory will be shared - to learn more check out Virtual Machine Memory Sharing in http://www.vmware.com/pdf/vi3_35/esx_3/r35u2/vi3_35_25_u2_resource_mgmt.pdf

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