When configuring HA, you also have to enable and configure admission control as otherwise there is no assurance that all virtual machines in a cluster can be restarted after a host failure. While won’t go too much into details about the different admission control policies (refer to Duncan’s and Frank Denneman’s blogs), I most often chose for admission control based on a percentage of cluster resources reserved for failover capacity as this according to my opinion is the most flexible option out of the three.

Not so long ago Frank Dennemanwrote a very good article explaining the admission control based on a percentage therefore I am not going to repeat what has been written already, however I do want to share the way I usually decide on how to set the percentage, have a look at the following table:

So what I am doing here is, basically a cluster can exists out of a maximum of 32 ESX hosts and let’s say every ESX host is equal to 100 Units. As by this example in the above table I decided that for a cluster with less than eight ESX hosts I will reserve 100 Units (meaning I will reserve one ESX host) for redundancy purposes and for a cluster with eight or more ESX hosts I will reserve 200 Units (two ESX hosts). By using the following calculation you can easily determine to what you have to set the percentage: “(number of host failures * total units provided by one ESX host) / total units within a cluster = percentage of resources reserved for failover“.

As the total cluster resources will change over time because hosts will be added (or removed), I developed a powershell script that either you launch on a interval or let it run based on an event that indicates that the cluster resources has changed:

param(
[string]$cluster
)

function connectToVI
{
 $vcs = $vCenters
 foreach ($vcenter in $vcs)
   {
     Write-Host "`nConnecting to: " $vcenter -BackgroundColor DarkGreen -ForegroundColor Black
     $vcHost = Connect-VIServer -Server $vCenter -User $vmxVCUser -Password $vmxVCPassword -ErrorAction SilentlyContinue
     if ($vcHost.Isconnected -eq $true)
     {
       Write-Host "`nsuccessfully connected to: " $vcenter
     }
     else
    {
      Write-Host "`nfailed to connect to: " $vcenter
      $vCenters = @($vCenters | Where-Object {$_ -ne $vCenter})
    }
   }
   return $vCenters
}

function disconnectFromVI()
{
 $vcs = $vCenters
 foreach ($vcenter in $vcs)
 {
   Write-Host "`nDisconnecting from: " $vcenter -BackgroundColor DarkGreen -ForegroundColor Black
   DisConnect-VIServer -Server $vCenter -Confirm:$false -ErrorAction SilentlyContinue
   $vCenters = @($vCenters | Where-Object {$_ -ne $vCenter})
 }
 Return $vCenters
}

 $vCenter = "vCenter Server"

 if ((Get-PSSnapin "VMware.VimAutomation.Core" -ErrorAction SilentlyContinue) -eq $null)
 {
   Measure-Command {Add-PSSnapin VMware.VimAutomation.Core -ErrorAction SilentlyContinue}
 }

 $vCenters = New-Object System.Collections.ArrayList
 [void]$vCenters.Add($vCenter)

 $vCenters = @(connectToVI)
 if ($vCenters.Count -gt 0)
 {
   if ($cluster.length -eq 0)
   {
     $cluster = (Get-VMHost -ID ("HostSystem-" + $VMWARE_ALARM_TARGET_ID) | Get-Cluster).Name
   }

   if ($cluster -ne $null)
   {
     $clusterview = Get-View -ViewType "ClusterComputeResource" -Filter @{"Name" = $cluster}
     $failoverCapacityInPercentage = $clusterview.configuration.dasConfig.admissionControlpolicy.cpuFailoverResourcesPercent

     if ($clusterview.configuration.dasConfig.admissionControlEnabled)
     {
       $hostCount = (@(Get-View -SearchRoot $clusterview.MoRef -ViewType "HostSystem")).Count

       if ($hostCount -gt 7 )
       {
         $failoverCapacityInPercentage = [math]::Round(((2* 100) / ($hostCount * 100))*100, 0)
       }
       else
       {
         $failoverCapacityInPercentage = [math]::Round(((1* 100) / ($hostCount * 100))*100, 0)
       }

       $clusterConfigSpec = New-Object VMware.Vim.ClusterConfigSpecEx
       $clusterConfigSpec.dasConfig = New-Object VMware.Vim.ClusterDasConfigInfo
       $clusterConfigSpec.dasConfig.admissionControlpolicy = New-Object VMware.Vim.ClusterFailoverResourcesAdmissionControlPolicy
       $clusterConfigSpec.dasConfig.admissionControlpolicy.cpuFailoverResourcesPercent = $failoverCapacityInPercentage
       $clusterConfigSpec.dasConfig.admissionControlpolicy.memoryFailoverResourcesPercent = $failoverCapacityInPercentage

       Write-Host "Setting Failover Capacity percentage to: $failoverCapacityInPercentage"
       $clusterview.ReConfigureComputeResource_Task($clusterConfigSpec, $true) | Out-Null
     }
   }
   $vCenters = @(disconnectFromVI)
 }