3 Replies Latest reply on Mar 12, 2018 8:42 AM by Martin Gavanda

    Need clarification in vSphere HA Cluster

    nhharshan Lurker

      I have a HA Cluster with three ESXi Hosts. All are running ESXi 5.5 which are managed by vCenter 5.5

       

      My HA configuration is as below for that cluster.

      Host Monitoring Enabled.

      Admission Control: Enable

      Admission Control Policy: Host failures the cluster tolerates = 1

       

      My Cluster "Resource Allocation" tab shows as below.

       

      vSphere-HA-Resource-Allocation.png

      Does this mean, 48704 Mhz of CPU and 385469 MB of RAM are always reserved and never allocated to any VM? Those will be used only when one host fails?

       

      All the three hosts in the cluster is now using more than 90% of CPU all the time. We cannot optimize the VMs as of now due to the design of the applications and we have not CPU over-committed on any VMs. For example, this is the status of one ESXi CPU utilization and that is almost same in all three.

       

      vSphere-ESXi-CPU.png

       

      In this situation,

      If I disable HA, will I get the reserved CPU and Memory back to the ESXi hosts? If one host fails all the VMs will still be moving and running with two ESXi hosts (I know there is a downtime)??

       

      Can someone clarify in detail?

        • 1. Re: Need clarification in vSphere HA Cluster
          daphnissov Guru
          Community WarriorsvExpert

          Does this mean, 48704 Mhz of CPU and 385469 MB of RAM are always reserved and never allocated to any VM? Those will be used only when one host fails?

          Yes, because you've selected that admission control should reserve one host worth of resources. It will therefore fail to power on any new VMs in that cluster that attempt to consume those reserved resources.

           

          In this situation,

          If I disable HA, will I get the reserved CPU and Memory back to the ESXi hosts? If one host fails all the VMs will still be moving and running with two ESXi hosts (I know there is a downtime)??

          You will get most of them back, however that means if you have host that fails, the other will probably not be able to run all the failed VMs. And, regardless of your changing the admission control settings, if all three of your hosts in this cluster are 90% utilized, you need more hosts anyway as that cluster is too highly utilized to ensure good performance.

          • 2. Re: Need clarification in vSphere HA Cluster
            nhharshan Lurker

            Unfortunately I do not have a test environment to test it. Can someone try it in real time and let me know. I want to be sure, that the resources shows as Reserved will be gotten back if I disable HA.

            • 3. Re: Need clarification in vSphere HA Cluster
              Martin Gavanda Enthusiast

              In this situation I would suggest disabling Admission Control Policy but keep HA active.

               

              In case of host failure, VMs will be restarted but there will be some congestion in terms of resources (app will be slower lets say) but me peronaly I would rather have all VMs up and running with slow performance (and I can decide what to do next - shutdown some of them?)