Which Windows Server Virtualization 08 (code named Viridian) storage is best for you?

If you have had a chance to play around with our prerelease bits you might be wondering what all the types of storage are.  You might also be wondering how to decide which storage is right for your virtual machine (VM).  One question I'll address in a future blog posting is storage performance.  Its a little early to talk performance with prerelease bits but rest assured we are working hard to drive much better performance.

  So what type of storage is there?

  • (dynamic / Fixed) Virtual Hard Drive (VHD) - This is a file stored in NTFS that looks like hard drive to the VM

  • Delta VHD - This stores all changes relative to a base VHD.

  • Pass-through drives - Pass-through drive are physical disks are exclusively assigned to the VM. 

There is also another dimension to consider

  • Synthetic Devices - These are new with Viridian.  These devices communicate over VMBUS (future post) with the root (sometimes called the host) partition.  These devices have an advantage over emulated devices because they better leverage the existing Windows driver architecture.

  • Emulated Devices - These devices try to emulate well know devices like disk, video, etc.  These are less performant than Synthetic Devices because of their emulation.

 An overview of pros and cons…

Storage Type



Dynamic VHD

Don’t have to decide the size of the disk upfront

Not as performant as Fixed VHD

Fixed VHD

Best performance for file based hard drives

Fills up

Delta VHD

Great for deployment of base golden image

Read might have to access delta and fixed drive + more file overhead.


Attach a physical drive / LUN to a VM


Does not support Virtual Machine Snapshots + no easy as easy to move as a VHD.  This is great for direct attached storage however.


Tricky configuration.  You have to take the drive offline in the root partition before it can be configured to a VM.  Use Disk Manager (diskmgmt.msc), diskpart.exe, …


So which storage is best for you?  You’ll like this answer – It depends J

We expect Fixed VHD and Pass-through to be the most common in deployment because of their predictable nature.

[[Since this is a CTP all these behaviors might change and there are no promises or warrantees it will continue to work this way]]


Comments (9)
  1. Piethein Strengholt says:

    How do you configure these Synthetic Devices? I got Hyper-V running on my laptop and most of the things seems to be fine. I haven’t seen the Synthetic Devices option anywhere….

  2. Tony Voellm says:

    The Synthetic devices are part of the "guest components".  In the prerelease bits there is an .iso that include them.  Simple mount the .iso and they should auto install.

  3. There are two types of disk controllers that Hyper-V supports: SCSI and IDE. There are two IDE controllers

  4. There are two types of disk controllers that Hyper-V supports: SCSI and IDE. There are two IDE controllers

  5. Ok. This is what I've learned so far about device access in Hyper-V. For device access in Hyper-V

  6. Hyper-V Performance FAQ Anthony F Voellm (aka Tony) 6/5/2008 http://blogs.msdn.com/tvoellm Q: What is

  7. Robie says:

    What about using the iSCSI initiator directly from the virtual server to the SAN?

    We have a very robust NetApp FAS-3020. From our other iSCSI initiator connections (on physical servers) and servers that are direct booting from the SAN via iSCSI HBA, we are getting IOPS of approx 20,000 or better per second and 95MBytes per second of data throuput (and higher) using MPIO and Round-Robin connections to the san over multiple iSCSI dedicated nics… (faster than 4mb fiber channel and easily matching direct attached SAS).

    With this in mind, is it better for us to use the pass-through to the Virtual Server HOST machine or to directly connect each virtual using the Microsoft iSCSI initiator (within the virtual)?




    [Tony’s reply]



    iSCSI works great in the guest.  iSCSI packets have to traverse the networking stacking both in the guest and the host.  Passthrough only does the NTFS stack on the guest and disk stack on the host.  The storage paths are much shorter (~4x) over networking.

     This means passthrough will be faster.


  8. Hyper-V Perfies, In a previous blog entry I explained the different types of storage choices you have

  9. Now that Hyper-V has been in the market for over 9 months a common question that has come my way is “what

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