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Matt McSpirit on Virtualisation, Management and Core Infrastructure

New video: Hyper-V and Pass-Through Disks

New video: Hyper-V and Pass-Through Disks

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Video #9 in the Windows Server 2008 Hyper-V series, is this video focussed on the use of Pass Through Disks with Hyper-V.  All the videos so far have used the .vhd file format for the virtual machines, of which there are 2 types; dynamic, and fixed.  A dynamic VHD is something you specify to have a maximum size, say, 127GB, and yet it starts at 0kb in size.  When you install a guest OS, or drop files into this dynamic VHD, it grows in size to reflect the size of the contents.  If we installed an OS inside a VM with a 127GB dynamic VHD as its base, the guest OS would believe it has 127GB space available to it, and yet in the physical world, the actual WindowsOS.vhd file that’s sat on my machine is only, say, 6GB, as this is the size of the installed files.

Fixed size VHDs on the other hand, are more of a buy-now-pay-now type scenario.  You specify you want a fixed size VHD, of 50GB in size, and Hyper-V creates a .vhd file of 50GB in size, which takes quite a while, as the whole 50GB has to be zero’d out which is great from a security perspective, but pants from a time consuming perspective.  You also have a single 50GB file on your hands!  Bit too big for a small USB thumb drive, so not as portable!

Fixed size VHDs are also more performant that their dynamic counterparts, in some cases, significantly more, and you can read all about that here:

The most performant type of storage for a virtual machine however, is physical storage.  What?  With Pass Through Disks, you can provide a raw piece of unformatted storage, so a physical local hard disk, a LUN from the SAN etc, to a virtual machine for it’s own exclusive read/write I/O, and judging by articles like this, the performance is smoking hot.

So anyway, the video:

The video is about 20 minutes long, but walks you through the use of Pass Through Disks in both non-clustered, and clustered scenarios.  You can watch it above in the browser, or alternatively, download the episode to take offline (right click, save-as), or watch all my videos, over at VBTV!

Enjoy, and feedback welcome as always!  Next up – maybe some 2008 R2 loveliness… :-)

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  • Nice work Matt on the Virtualboy TV site Videos, came across these via link in TechNet newletter.



  • Hi Steve,

    Thanks for this!  It's all part of the service :-)


  • Just seen this video, a great overview of what a Pass Through disks are and when to use them

  • Thanks!

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