I have a bad habit of making the boot drives for my virtal machines quite small, usually around 16GB. My Exchange 2010 server is virtualised and has this sort of configuration, a 16-gigabyte C: drive which just the OS, and a D: drive with Exchange and the databases on it. At some point, Windows decided to increase the swapfile to a point where I was uncomforable with the amount of free space on C: drive. No problems I thought, I’ll put it on D: drive. It would refuse to take it and would recreate the swap file on C: as a temporary swap file leading to bad performance.
After a fair amount of hair pulling and bad performance, I found out the issue. As detailed here, you can’t boot a Hyper-V machine from a SCSI Virtual Disk (that is, a virtual hard disk created and attached to the VM’s SCSI controller), not can you create a swapfile on it. This is apparently caused by the nature of the SCSI disk and controller and can be fixed by using an IDE virtual hard disk instead. So now the Exchange server has a 3rd drive, S: drive which is a 32-gigabyte IDE virtual disk that happily stores the swap file. Performance is much improved as a result.