EVC enabled clusters?

[trink408]

Are you guys configuring your clusters with EVC enabled or disabled?

Just curious if you lose any functionality/performance by enabling EVC on your clusters?

Thanks for your time

Kevin

[a_p_]

Due to the fact that there are often different pieces of hardware (CPUs) which have to run in a cluster, EVC is very helpful feature. I didn't do any performance tests regarding differences with and without EVC enabled, but from what I read so far there is not much you need to be worried about.

Maybe http://kb.vmware.com/kb/1005764 helps answering your question.

André

[peterdabr]

Hi,

We do enable EVC for our customer clusters that we manage. Frequently, clients requests additional hosts added to the cluster to increase resource capacity and often times those new hosts have slighthly different CPU feature set within  the same CPU category. When that happens and no EVC is enabled, live vmotioning VMs onto  new hosts errors out because of incomatibility in instructions sets between source esx and dest esx. One of the workarounds is to mask CPU bits at the VM level, though it can't be done while VM is up, hence a major problem for clients that require their vms to stay up 99.9% of the time.

EVC always brings the CPU base for the cluster to the host with the 'lowest' instruction set so there's always a risk of loosing some CPU functionality and/or performance but it usually do not stand out.

Peter D

[trink408]

Thanks guys.

I wasn't a part of our original vsphere build, and the consultants that setup the first cluster never enabled EVC. I was curious if their was good reasoning not to do so. The way it sounds I really won't lose anything, and I know it will help with vmotion going forward as we add newer hardware.

Now if I were to change the EVC mode on the existing cluster, would I have to power off all the vm's first?

[peterdabr · Accepted solution]

No. When enabling it, you'll be given a list of capatible EVC modes that your could choose from based on the CPU variety in the cluster and you should choose the highest mode.

Peter D

[chriswahl]

VMware has made it very clear with a number of white papers and technical articles that enabling EVC has no performance impact.

With regards to enabling EVC: Sometimes you do have to power off a running VM, but that really depends. As long as none of the VMs on the host are running instructions not supported by the EVC baseline, you're fine. Otherwise, you'll see a warning when you select the EVC mode.

Per the VMware KB:

Can I enable EVC on an existing non-EVC enabled cluster with powered-on virtual machine guests?

For information related to vCenter Server 5.0, see CPU Compatibility and EVC in the vCenter Server and Host Management Guide.

[trink408]

So from the drop down list of EVC modes, just keep changing until you find one that says Validation Succeeded?

Only one of the optional EVC modes says Validation Succeeded, the rest tell me I cannot use them..

[peterdabr]

Yes, keep going up the dropbox list until you find the first one that passes validation.

[rickardnobel]

Chris Wahl wrote:

VMware has made it very clear with a number of white papers and technical articles that enabling EVC has no performance impact.

That is my opinion too. Since only newly added x86/x64 instructions are masked, but the rest of the CPU "remains the same", i.e. speed in Mhz, number of cores, amount of L1, L2 cache and so on - will not change. There is a large chance that the masked out instructions are never needed at all for the VM servers running, even if they were available.