Understanding VMware EVC
Setting up VMware EVC is sort of like tuning your car engine. You do not want to do much of the work while the car is running. The same holds true for EVC. Specifically in order to enable VMware EVC you need to set up the system BIOS properly. One setting is well known yet the other is not so well known.
For HP hardware enable the following BIOS options on Intel processors. There is similar options for AMD processors
- enable Intel-VT within the BIOS
- enable No Execute Memory Protection within the BIOS. This option has as many names as there are hardware vendors but all contain either X-Bit or No Execute in the name.
Once you have the BIOS enabled properly and the host rebooted you next need to enable EVC, which you can not do if there are VMs running on the host. For some this causes some issues as VirtualCenter is running as a VM. In this case you need to do some creative VMotioning to move VMs to a host that is currently NOT in EVC mode.
Penultimately, it is important to realize that the BIOS changes, change the behavior of the CPU, and in order to move VMs to an EVC enabled node, you may have to do a cold migration. This implies powering off VMs. Sometimes this is not necessary if you already had Intel-VT enabled for example. But you will have to VMotion between like CPUs to make this happen without needed to set CPU masks which you will have to unset once the VMs are in the EVC cluster.
Lastly, VMware EVC only removes the need to set per VM CPU Masks for the same global family of processors. I.e. Intel and AMD. You still need to use CPU Masks if you want to VMotion VMs from an Intel host to an AMD host. But within the same family the CPU Masks are not required.
New hosts to be added to an EVC Cluster just require the BIOS changes and no active VMs. It is the first couple of hosts that currently pose the most problems.
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A few refinements for the AMD side
The AMD equivalent to Intel-VT is AMD-V but you do not need to have it enabled for EVC. We have an EVC cluster of HP DL585 proliant G1/G2/G5 servers and AMD-V is not an option on a G1. If you turn it on for G2 and G5 then you introduce CPU incompatibility with the G1 and VMotion fails.Happily the CPU mask default for G1 instructions matches the EVC cluster mask so we can just VMotion VMs that are running outside the EVC cluster on G1 servers directly into the EVC cluster.
Unfortunately the same is not true for G2 or G5 and you cannot change the mask in virtual center anymore since VC2.5u2 while the VM is running. Cold migrations are the answer so far.
EVC is a good step forward in compatibility, even if the transition is a little rough in spots.
Thank you for the article and your ESX book and other articles.
EVC processor support
VMware has a KB article listed the processors that are supported with EVC. The URL is http://kb.vmware.com/kb/1003212.EVC is an interesting
EVC is an interesting features and I would like to share how to enable EVC on ESX 3.5 with NO Downtime wrote sometimes ago http://malaysiavm.com/blog/how-to-enable-evc-on-esx-35/