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LoadBalanceVMsInCluster.ps1

Updated: February 4, 2010

Applies To: Virtual Machine Manager 2008 R2, Virtual Machine Manager 2008 R2 SP1

System Center Virtual Machine Manager (VMM) does not automatically perform load balancing among the hosts in a host cluster. However, by using the memory properties of the hosts and virtual machines in the cluster, you can calculate whether virtual machines should be migrated to other hosts in order to perform load balancing among the cluster nodes.

The following script gets the host in the cluster with the most available memory and the host with the least available memory. The script determines whether the virtual machine that uses the most memory on the host with the highest memory usage should be migrated to the host with the lowest memory usage to balance the load. If this calculation returns a result of False, the script gets the next virtual machine in the array and repeats the calculation until either a result of True is returned, or it has looped through all the virtual machines in the array. If the calculation returns a result of True, the script migrates the virtual machine to the host with the lowest memory usage. To help ensure successful migrations, the script migrates virtual machines only to hosts with a host rating greater than or equal to three. If the host rating is lower than three, the script exits. If the host has a host rating of zero, the script gets the next best destination host and migrates the virtual machine. If there are no more hosts, or if the host rating of the new host is less than three, the script exits. The script loops through the process from the beginning of the script until the virtual machines are balanced across the cluster.

Disclaimer


# Filename:      LoadBalanceVMsInCluster.ps1
# Description:   Load balances virtual machines in a cluster by
#                calculating the amount of memory on the hosts with
#                the memory used by the virtual machines to
#                determine whether the virtual machines should be 
#                migrated.

# Get the Virtual Machine Manager server.
Get-VMMServer -computername "VMMServer01.Contoso.com" | out-null

# Get the cluster.
$Cluster = Get-VMHostCluster -name "Cluster01.Contoso.com"

# Create an array to store the names of the virtual machines 
# that cannot be migrated. 
$FailedVMs = @()

# Create a loop to repeat the process.
Do
{
   # Get the Hosts in the cluster, and then get the host with
   # the most amount of available memory and the host with the 
   # least amount of available memory.

   $vmhosts = @(Get-VMhost -VMHostCluster $cluster | sort-object -property availablememory -descending)
   $h=0
   $HostMostMem = $vmhosts[$h]
   $count = $vmhosts.count -1
   $HostLeastMem = $vmhosts[$count]

   # Get the virtual machines on the host with the least available memory.
   $VMs = @(Get-VM -vmhost $HostLeastMem | 
        Where-Object {$FailedVMs -notContains $_.Name }  |
        Sort-Object -property memory -descending)
    If (! $Vms) { break }

   $i=0
   $VM = $VMs[$i]

   # Create a function to calculate whether the amount of memory on the
   # host with the least amount of available memory plus the memory used
   # by the specified virtual machine is less than the memory on thehost 
   # with the most amount of available memory plus the memory used by the 
   # specified virtual machine.

   Function CalculateMemory ($Host1, $Host2, $VirtualMachine)
   {
      $MemoryCalc = ($HostLeastMem.availablememory+$VM.memory) -lt ($HostMostMem.availablememory-$VM.memory)
      $MemoryCalc
   }

   # Call the function to do the initial memory calculation.
   $MemoryCalc = CalculateMemory $HostMostMem $HostLeastMem $VM

   # Create a loop that recalculates the memory for each virtual machine
   # in the array while the memory calculation does not equal True.

   While ($MemoryCalc -ne $True)
   {
      If ($i -lt ($VMs.count-1))
      {
         $i = $i+1
         $VM = $VMs[$i]
         $MemoryCalc = CalculateMemory $HostMostMem $HostLeastMem $VM
      }
      Else 
      {
         Write-Host "The virtual machines are load balanced in the cluster."
         If ($FailedVMs) {Write-Host "The following virtual machines were not able to be migrated:" $FailedVMs}
         Break
      }
   }

   # If the memory calculation equals True, migrate the specified virtual
   # machine to the host with the most amount of available memory.

   If ($MemoryCalc -eq $True)
   {
      # Get the host rating for $HostMostMem to ensure that the migration will succeed.
      $HostRating = Get-VMHostRating -VM $VM -VMHost $HostMostMem -PlacementGoal "LoadBalance" -IsMigration

      # If the host rating for $HostMostMem is greater than or equal to 3, migrate the virtual machine.
      If ($HostRating.Rating -ge "3")
      {
         Write-Host "Migrating virtual Machine $VM from host $HostLeastMem to host $HostMostMem."

      # If the virtual machine cannot be migrated, display the error, add the virtual machine to the
      # $FailedMVs array, and then continue.
$err = @()
Move-VM -VM $VM -VMHost $HostMostMem -ErrorAction SilentlyContinue -ErrorVariable err| out-null
         If ($err) 
         {
            $err | Write-Error
            $FailedVMs += $vm.Name
         }

      }

      # If the host rating for $HostMostMem is zero, find the next host with the most available memory.
      ElseIf ($HostRating.Rating -eq "0")
      {
         $h = $h+1
         $HostMostMem = $vmhosts[$h]

         If ($HostMostMem = $HostLeastMem) 
         {
            Write-Host "There are no other hosts to which virtual machine $VM can be migrated."
            Return
         }

         $HostRating = Get-VMHostRating -VM $VM -VMHost $HostMostMem -PlacementGoal "LoadBalance" -IsMigration

         # If the host rating for $HostMostMem is greater than or equal to three, migrate the
         # virtual machine.
         If ($HostRating.Rating -ge "3")
         {
            Write-Host "Migrating virtual Machine $VM from host $HostLeastMem to host $HostMostMem."
            Move-VM -VM $VM -VMHost $HostMostMem | out-null
         }
         Else
         {
            Write-Host "There is no host with a high enough placement rating to load balance the virtual machines."
            Return
         }
      }

      Else
      {
         Write-Host "There is no host with a high enough placement rating to load balance the virtual machines."
         Return
      }
   }

   Else
   {
      Return
   }
}While ($MemoryCalc -eq $True)
# Loop to repeat the process until the cluster is load balanced.

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