Fixed Disks vs. Physical Disks

I recently was setting up a Home Server on top of Hyper-V (Note: this is not a supported configuration, so if you wish to do the same - be it on your head) and had to decide between using physical disks directly attached to the virtual machine, or using fixed size virtual hard disks.  I had 5 SATA disks that I wanted to connect to the Home Server - and in the end I decided to use fixed size virtual hard disks where each physical disk had a single fixed size virtual hard disk that took up all the space available.

There were a number of things that I considered before coming to this decision:

1. Performance: There is a big performance difference between dynamically expanding virtual hard disks and fixed size virtual hard disks.  But there really is not much difference between fixed size virtual hard disks and physical disks, so this did not really influence my decision here.
2. Mobility: This is an interesting one, as it can go either way depending on your hardware.  In my case - using direct attached SATA storage - fixed size virtual hard disks are easier to move around.  If - however - you are using network based storage or SAN infrastructure, then physical disks are easier to move around.
3. Backup: This was a big one for me.  If you use physical disks you cannot use VSS to backup your virtual machine - but you can if you are using fixed size virtual hard disks (similarly - you cannot use virtual machine snapshots with physical disks - but using virtual machine snapshots with fixed size virtual hard disks is not a good idea either).
4. Data Safety: I spent a while pondering this one.  The question is which option is least likely to be significantly affected by random data corruption.  As I discussed a little while ago the fixed size virtual hard disk format is mostly just data - and corruption there would have the same effect as for a physical hard disk.  When using a fixed size virtual hard disk you would have to have corruption in the 511 byte footer, or in the parents NTFS file system structure, to cause a problem.  So while there is definitely a greater potential for "catastrophic" data loss - the difference is fairly small.
5. Hardware monitoring: This was another big one for me.  By using fixed size virtual hard disks I can run hardware monitoring tools (like SMART monitoring tools) in the parent partition.  If you use physical disks with a virtual machine you cannot use these sorts of tools in the parent partition or in the virtual machine.

Hope this helps you when you are faced with similar decisions.

Cheers,
Ben

Comments

• Anonymous
  January 31, 2009
  Hello, I have always been wondering if it is possible to run Home Server under Hyper-V. This is really my preferred setup as a Home Server normally is very under utilized and I would like to have another OS for maybe a mail and web server running in parallell. I know this isn't supported, so therefor I have never really tried it. But since you are perhaps you could shed some light on what problems you found so far? Do you have any problems with the dynamic storage in HS?

• Anonymous
  February 06, 2009
  Hi, I have a doubt on the efficience of the way Hyper-V creates empty fixed virtual disks. Instead of create a big file of 'n' gigabytes it starts with a little file and make it grow by steps of 256 KB until the desired size. So, I did a test: From the SCVMM server, using two Powershell consoles, I launched simultaneously two command to create 2 fixed virtual disks... The result is what I guessed: the VHD files are fragmented... When you make a P2V from a physical machine, Hyper-V creates immediatly a big VHD file and then starts to transfer data in it. Why does Hyper-V create empty VHD fixed files this way ? Why is it different with P2V ? Thanks for any help. Regards, Bruno

• Anonymous
  February 19, 2009
  Niclas Lindgren - I have had no problems whatsoever with this configuration. bpaci - Check: http://blogs.msdn.com/virtual_pc_guy/archive/2008/12/10/why-does-it-take-so-long-to-create-a-fixed-size-virtual-hard-disk.aspx Cheers, Ben

• Anonymous
  February 19, 2009
  Hi Ben, I've already checked the blog about the time it takes to create a fixed disk... My question is not "why it takes time" but it is "why the VHD file starts with a 0 bytes size and grow until the 10 GB desired size"... Why not create a 10 GB file and they fill it with zeros ? Making the file grow until the desired size will result in fragmented file if there is another writing on the same physical disk at the same time. If you reproduce the test I explained in my previous message you will see with your own eyes that the two VHD files are fragmented. They look like interlaced. Regards Bruno

• Anonymous
  November 25, 2010
  Just stumbled upon this by googling - thanks for sharing the considerations, I'll follow suit when creating my setup.