Virtualized SharePoint solutions became very popular after Microsoft officially announced support of VPC, Hyper-V and 3rd party virtualization for SharePoint farms. So, number of virtualized implementations increased. But, there is one trap, which you need to avoid – is installing SharePoint on virtualized Windows 2008 Server with Domain Controller role.
There is nothing bad with SharePoint and DC together, except that the best practices recommends you not to have them together on one box. The actual problem in performance of hard drive when you activate DC in virtualized Windows 2008 Server.
After DC is activated, Windows turn off any hard drive caching and have direct reads/writes to keep AD integrity
“To help preserve the integrity of the Active Directory database if a power loss or another failure were to occur, the Active Directory directory service performs unbuffered writes and tries to disable the disk write cache on volumes hosting the Active Directory database and log files. Active Directory also works in this manner when it runs in a virtual hosting environment.”
More details in Microsoft Knowledge Base article 888794
So, any installations to DC Windows 2008 server will be not effective at all. DC must be separate box, without any others applications.
PS: This behavior is applicable only for virtualized Windows 2008, not physical installation.