Windows Azure Guest OS

In a Windows Azure project, you can specify the Guest OS version for your VM.  This is done by setting the osVersion property inside the ServiceConfiguration file:

If you don’t specify a version explicitly, the latest Guest OS is chosen for you.  For production applications, it’s probably best to always provide an explicit value, and I have a real world lesson that demonstrates this!

MSDN currently has a Guest OS Version and SDK Compatibility Matrix page that is extremely helpful if you’re trying to figure out which versions offer what features.  I recently ran into a problem when examining some of my performance counters – they were all zero (and shouldn’t have been)!  Curious to find out why, I did some digging (which means someone internal told me what I was doing wrong).

In short, I had specified a performance counter to monitor like so:  "\ASP.NET Applications(__Total__)\Requests/Sec".  This had worked fine, but when I next redeployed some weeks later, the new Guest OS (with .NET Fx 4.0) took this to mean 4.0 Requests/Sec, because I didn’t specify a version.   So, I was getting zero requests/sec because my app was running on the earlier runtime.  This was fixed by changing the performance counter to "\ASP.NET Apps v2.0.50727(__Total__)\Requests/Sec".  

For more information on this, check out this article on MSDN.  And thanks to the guys in the forum for getting me up and running so quickly!