Thursday 1 December 2011

Paravirtualizing vSphere guests

Although just setting up a virtual machine on a ESX server works well there are ways to improve the VM's performance and reduce some of the CPU overhead required when the VM interacts with the virtual hardware. The two ways I have used are installing paravirtualized drivers for network interface and for the storage controller.

If you are installing Windows from media or an ISO image:
  • Edit the VM settings, replace any networks adapters with ones that are of the type "VMXNET 3."
  • Change the SCSI controller type to "Paravirtual" 
  • On the floppy drive choose "use existing floppy image in datastore:", click browse, then find the relevant image in vmimages\floppies for your version of windows. Dont forget to "select connect at startup"
  • Hit F6 during the initial installation to add the paravirtual SCSI drivers from the floppy image. 
  • The OS wont recognise the network card until the VMware tools are installed.
If the VM is already set up and running Windows with VMware tools the drivers for the paravirtualised SCSI adapter are also installed with VMware tools but you will need to add a second SCSI adapter of the type "Paravirtual" and boot before Windows will be happy to with the first (boot) SCSI adapter being set to "Paravirtial."
  • Edit the VM settings, replace any networks adapters with ones that are of the type "VMXNET 3"
  • Using the Vshpere client Edit the settings of your Windows VM, Click the "Add..." button, choose "Hard Disk", Choose "Create a new Virtual disk" and set a small size like 8 MB.
  • If your VM already has a SCSI controller you will need to add an additional one. To get another controller you will need to choose a virtual device node which is from the second SCSI controller. I usually just select SCSI(1:0).
  • Check device manager to ensure the the PVSCSI controller has been detected.
  • Shutdown the VM, remove the hard drive you have just added and change the remaining SCSI adapter to type "Paravirtual"
  • Start the machine, if all is well (you will see a blue screen if it isn't) you will have successfully changed windows to use a more efficient way to talk to it's boot drive.
Converting IDE drives to SCSI
I struggled with Windows XP box that I had imported using VMware Converter that only had an IDE drive. I tried deleting the hard drive (I was careful not to delete the underlying files), then I went to add the hard drive and set it to SCSI it would only allow IDE. I overcame this by shutting down the virtual machine, editing the relevant .vmdk file and changing the ddb.adapterType parameter to lsilogic like so:
ddb.adapterType = "lsilogic"
You should now be able to add the hard drive back in specifying the existing .vmdk file and set the SCSI adapter to "Paravirtual"

Debian 6
On a system that is already running and the VMware tools are installed. I ran all updates and it was simply a case of removing the existing network adapter and adding a new one of the type "VMXNET 3" and changing the SCSI adapter to "Paravirtual" and everything worked.

Update: Also check out this post on changing the IO scheduler on linux VMs squeezing more from your Linux VMs

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