Q: I'm using an application with a JET database engine with Hyper-V--is it OK to run it on a virtual disk attached to the IDE controller?
A. No. The virtual IDE controller doesn't guarantee I/O, which is required for many server workloads because of the use of write caching.
It's therefore recommended to place any database on a virtual hard disk (VHD) that's connected to the virtual SCSI controller, which provides IO guarantees needed for server workloads (this includes Active Directory Domain Controller databases).
Only use the IDE controller for the OS boot. With Hyper-V 2012 R2, this isn't a problem since the Generation 2 virtual machines (VMs) use only the virtual SCSI controller and can boot from virtual SCSI.