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

# Variables
$VCServerName = "MYVCSERVER"
$CustomFieldName = "LUNs"
$ManagedObjectType = "HostSystem"

# 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
$VMHosts = Get-VMHost
$VMHostViews = $VMHosts | Get-View
ForEach ($VMHostView in $VMHostViews)
	{
	$LUNCount = ($VMHostView.Config.FileSystemVolume.MountInfo | Measure-Object).Count
	# Compare value to current value
	If ($LUNCount -ne ($VMView.CustomValue | ?{$_.Key -eq $myCustomField.Key}).Value)
		{
		# Set Custom Value
		$VMHostView.setCustomValue($CustomFieldName,$LUNCount)
		}
	}
Disconnect-VIServer -Confirm:$False