Backup Hyper-V VMs using save-vm, export-vm, result-vm with basic rotation

Backup-VMs.ps1

$ErrorActionPreference = "Stop"

# https://gist.github.com/tomfanning/617bde27596de3b1ca62

$backuptarget = "b:\HyperV"
$compression = $false

$VMs = Get-VM

foreach ($vm in $vms){
  $vmname = $vm.Name
  Write-Host "Exporting $vmname"
  $vmtargetfolder = $backuptarget + "\" + $vmname  
  $vmPrevBackupFolder = $vmtargetfolder + "_prev"

  # remove leftover previous backup folder
  if (test-path $vmPrevBackupFolder){
    remove-item -recurse -Force $vmPrevBackupFolder
  }

  # rename current backup folder to previous
  if (test-path $vmtargetfolder){
    rename-item $vmtargetfolder $vmPrevBackupFolder
  }
  
  $running = $false
  if ($vm.state -eq "Running") {
	# save RAM to disk and pause VM
	save-vm -vm $vm
	$running = $true
  }
  
  $success=$false
  for($i=1; $i -le 3; $i++){
	try {
		# export running VM to current backup folder
		export-vm -vm $vm -path $backuptarget
		$success = $true
		break
	}
	catch {
		write-host "Retrying..."
		remove-item -recurse -force $vmtargetfolder
	}
  }
  
  if ($running) {
    # resume from saved state
    start-vm -vm $vm
  }
  
  if ($success -eq $false){
	throw "Fatal error exporting VM after 3 retries: $_.Exception.Message"
  }
  
  # apply compression
  if ($compression) {
	  write-host "Compressing..."
	  $cmd = "cmd /c compact /q /c /s:""$vmtargetfolder"""
	  cmd /c $cmd
  }

  # delete previous backup folder
  if (test-path $vmPrevBackupFolder){
    remove-item -recurse -Force $vmPrevBackupFolder
  }
}

write-host "All done"