Unable to start additional virtual machines when Hyper-V has plenty of resources

Ben Armstrong from the Hyper-V team just published a really interesting blog article about CPU Reserves.

The thing that you really need to think about here is how tweaking these settings impacts your virtualization environment as a whole – and Ben spends some time trying to relate that concept.

http://blogs.msdn.com/b/virtual_pc_guy/archive/2011/02/14/hyper-v-cpu-scheduling-part-1.aspx

In the forums I frequently advise folks to leave the knobs and buttons alone unless you have a really good reason for modifying them.  Why?  Because the system is designed to self manage and it does a really good job of it.

The place where you get into problems is not from tweaking a single VM on a single hypervisor.  This usually works.  The problems come when that VM is moved to another hypervisor, for any number of reasons.

After moving to another hypervisor the environment changes, how that VM interacts changes, the settings on the other VMs are different, and so on.

The big take away from Ben’s article is – if you change the settings, understand what you are doing and how it impacts the entire environment, not just this VM.

The little take aways: don’t change the settings unless you really need to, if you do change the settings be sure to document what and most importantly WHY, and tweak as few as absolutely possible.

Think on that a bit, and if you like play around a bit – it will all come clear once you create a conflict caused by a setting that the environment should be able to support.