Thursday 5 January 2012

VM Dynamic VHD Performance On Shared LUNs/Partitions

Many years back we did a lot of performance testing on NT4 and Windows Server 2000 on standalone SCSI and RAID based SCSI arrays for server swap file performance.

It was not uncommon for the swap file to get quite fragmented over time in our experience with no no utilities beyond very expensive third party ones that would help us get things back together.

We got into some very animated discussions with the lead technician out our number 1 client at the time because setting aside a partition for a dedicated swap file was _expensive_.

As a result of that experience we _always_ build all of our servers and high end workstations with a dedicated partition for the OS swap file and Adobe Scratch Files (sometimes with their own SSD).

We carried that experience into our current methodology for configuring the storage setup for a VM’s VHD(s).

Shared Storage and VHDs

When VHDs are going to be hosted on shared storage that will have a number VHDs we set up static VHDs using VHDTool. The VHD files remain contiguous thus avoiding performance degradation over time due to fragmentation that would be experienced with dynamic VHDs.

It is our preference to set up dedicated partitions/LUNs on shared storage for dynamic VHDs when they are required. This leaves room for expansion without concern for fragmentation.

We create dynamic VHDs where portability gives us the option to move the file around. For example we can copy 120GB instead of 1.5TB.

  • Rule of thumb for Hyper-V Storage: Total Storage – 1GB = Available for VHD(s).

We have seen the behaviour where a Hyper-V VM or VMs go into Paused-Critical mode due to storage being full.

So, we make sure that the sum total GB/TB of our static and/or dynamic VHDs stored on that LUN/Partition never exceed the total amount of storage available minus 1 Gigabyte.

Philip Elder
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book

*Our original iMac was stolen (previous blog post). We now have a new MacBook Pro courtesy of Vlad Mazek, owner of OWN.

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