Tuesday, January 09, 2018

Admission Control "Dedicated fail-over hosts"

More then three years ago I published the blog post about "vSphere HA Cluster Redundancy". There are three algorithms
  • Define fail-over capacity by static number of hosts
  • Define fail-over capacity by reserving a percentage of cluster resources
  • Use dedicated fail-over hosts
I discussed first two algorithms very well but the third one "dedicated fail-over hosts" was described briefly by following words ...
This algorithm simply dedicates specified hosts to be unused during normal conditions and used only in case of ESXi host failure. Multiple fail-over dedicated hosts are supported since vSphere 5.0. This algorithm will keep your capacity and performance absolutely predictable and independent on VM reservations. You'll get exactly what you configure.
I have been asked recently by one my customer for some details behavior of  "dedicated fail-over hosts" method.

Question #1: How will be VMs restarted in case of single host failure when two hosts are dedicated for fail-over?

Answer: All impacted VMs are restarted and spread across both dedicated fail-over hosts.

Question #2: Is it possible to vMotion VMs to dedicated hosts for fail-over?

Answer: Of course not. These hosts are dedicated just for fail-over and vSphere Cluster is aware about it, therefore, it will not allow administrator nor DRS to migrate VMs there.

Question #3: What will happen with VMs when the failed host is back?

Answer: VMs will stay on on hosts dedicated for fail-over unless DRS will move them to other hosts. Based on my testing, DRS will do it probably just in case of lack of resources, therefore, some VMs can stay on dedicated fail-over hosts, which is not good. Therefore, vSphere administrator should check the cluster state after host failure and move all VMs out of dedicated fail-over hosts if DRS did not do it before.

Hope this helps broader VMware community to better understand VMware Admission Control. And as always, if you have some other question, opinion or different experience, please, feel free to leave the comment below.

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