##############################
# Script created by Hugo Peeters #
# http://www.peetersonline.nl   #
##############################

# Variables
$VCServerName = "MYVCSERVER"
$CustomFieldName = "RDM Size (GB)"
$ManagedObjectType = "VirtualMachine"

# Script
$VC = Connect-VIServer $VCServerName
$SI = Get-View ServiceInstance
$CFM = Get-View $SI.Content.CustomFieldsManager

$myCustomField = $CFM.Field | Where {$_.Name -eq $CustomFieldName}
If (!$myCustomField)
	{
	# Create Custom Field
	$FieldCopy = $CFM.Field[0]
	$CFM.AddCustomFieldDef($CustomFieldName,$ManagedObjectType,$FieldCopy.FieldDefPrivileges,$FieldCopy.FieldInstancePrivileges)
	}
	
# Fill Custom Fields
$VMs = Get-VM
ForEach ($VM in $VMs)
	{
	$RDMSize = [math]::Round((($VM | Get-HardDisk | Where {$_.DiskType -ne "flat"} | Measure-Object CapacityKB -Sum).Sum * 1KB / 1GB),0)
	$VMView = $VM | Get-View
	# Compare value to current value
	If ($RDMSize -ne ($VMView.CustomValue | ?{$_.Key -eq $myCustomField.Key}).Value)
		{
		# Set Custom Value
		$VMView.setCustomValue($CustomFieldName,$RDMSize )
		}
	}
Disconnect-VIServer -Confirm:$False