Optimize Hyper-V VM Placement To Match CSV Ownership

This post shares a PowerShell script to automatically live migrate clustered Hyper-V virtual machines to the host that owns the CSV that the VM is stored on. The example below should work nicely with a 2-node cluster, such as a cluster-in-a-box.

For lots of reasons, you get the best performance for VMs on a Hyper-V cluster if:

  • Host X owns CSV Y AND
  • The VMs that are stored on CSV Y are running on Host X.

This continues into WS2016, as we’ve seen by analysing the performance enhancements of ReFS for VHDX operations. In summary, the ODX-like enhancements work best when the CSV and VM placement are identical as above.

I wrote a script, with little bits taken from several places (scripting is the art of copy & paste), to analyse a cluster and then move virtual machines to the best location. The method of the script is:

  1. Move CSV ownership to what you have architected.
  2. Locate the VMs that need to move.
  3. Order that list of VMs based on RAM. I want to move the smallest VMs first in case there is memory contention.
  4. Live migrate VMs based on that ordered list.

What’s missing? Error handling 🙂

What do you need to do?

  • You need to add variables for your CSVs and hosts.
  • Modify/add lines to move CSV ownership to the required hosts.
  • Balance the deployment of your VMs across your CSVs.

Here’s the script. I doubt the code is optimal, but it works. Note that the Live Migration command (Move-ClusterVirtualMachineRole) has been commented out so you can see what the script will do without it actually doing anything to your VM placement. Feel free to use, modify, etc.

#List your CSVs 
$CSV1 = "CSV1" 
$CSV2 = "CSV2"

#List your hosts 
$CSV1Node = "Host01" 
$CSV2Node = "Host02"

function ListVMs () 
{ 
    Write-Host "`n`n`n`n`n`nAnalysing the cluster $Cluster ..."

    $Cluster = Get-Cluster 
    $AllCSV = Get-ClusterSharedVolume -Cluster $Cluster | Sort-Object Name

    $VMMigrationList = @()

    ForEach ($CSV in $AllCSV) 
    { 
        $CSVVolumeInfo = $CSV | Select -Expand SharedVolumeInfo 
        $CSVPath = ($CSVVolumeInfo).FriendlyVolumeName

        $FixedCSVPath = $CSVPath -replace '\\', '\\'

        #Get the VMs where VM placement doesn't match CSV ownership
        $VMsToMove = Get-ClusterGroup | ? {($_.GroupType –eq 'VirtualMachine') -and ( $_.OwnerNode -ne $CSV.OWnernode.Name)} | Get-VM | Where-object {($_.path -match $FixedCSVPath)} 

        #Build up a list of VMs including their memory size 
        ForEach ($VM in $VMsToMove) 
        { 
            $VMRAM = (Get-VM -ComputerName $VM.ComputerName -Name $VM.Name).MemoryAssigned

            $VMMigrationList += ,@($VM.Name, $CSV.OWnernode.Name, $VMRAM) 
        }

    }

    #Order the VMs based on memory size, ascending 
    $VMMigrationList = $VMMigrationList | sort-object @{Expression={$_[2]}; Ascending=$true}

    Return $VMMigrationList 
}

function MoveVM ($TheVMs) 
{

    foreach ($VM in $TheVMs) 
        { 
        $VMName = $VM[0] 
        $VMDestination = $VM[1] 
        Write-Host "`nMove $VMName to $VMDestination" 
        #Move-ClusterVirtualMachineRole -Name $VMName -Node $VMDestination -MigrationType Live 
        }

}

cls

#Configure which node will own wich CSV 
Move-ClusterSharedVolume -Name $CSV1 -Node $CSV1Node | Out-Null 
Move-ClusterSharedVolume -Name $CSV2 -Node $CSV2Node | Out-Null

$SortedVMs = @{}

#Get a sorted list of VMs, ordered by assign memory 
$SortedVMs = ListVMs

#Live Migrate the VMs, so that their host is also their CSV owner 
MoveVM $SortedVMs

Possible improvements:

  • My ListVMs algorithm probably can be improved.
  • The Live Migration piece also can be improved. It only does 1 VM at a time, but you could implement parallelism using jobs.
  • Quick Migration should be used for non-running VMs. I haven’t handles that situation.
  • You could opt to use Quick Migration for low priority VMs – if that’s your policy.
  • The script could be modified to start using parameters, e.g. Analyse (not move), QuickMigrateLow, QuickMigrate (instead of Live Migrate), etc.

6 thoughts on “Optimize Hyper-V VM Placement To Match CSV Ownership”

  1. Does not work for me if only 1 VM is on the wrong host. If more than one VMs are effected then the script works fine!

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