I've been reading up on transitioning my SBS 2011 server to a 2012 R2 Essentials server. I found one article that suggests that in a virtualized AD server, one should not store the Active Directory database (NTDS) onto the system drive. The article doesn't site any reason why.
Warning: If you are deploying this server as a virtual machine, it is recommended that you store the AD database & SYSVOL files on a non-system volume (e.g. E:\ instead of C:). 1
Is this in fact the recommendation? If it was at some point is it relevant today? My Hyper-V host is Hyper-V Server 2012 R2, and SBS 2011 is already running as a virtual.
You can store the AD database and logs on either the system drive or on a separate data drive if you want to. Both are fully supported configurations. In older hardware, it was often advisable to store the database and log files on a separate data drive for performance reasons, so that those files would have a "dedicated spindle(s)." But in today's hardware with very fast SSDs the performance benefit is often not really noticeable. Also if the DC is virtualized and the "data drive" and the "system drive" would both be carved from the same underlying shared storage, there's no real performance win there either. It's also not a big deal if you have enough RAM to keep most or all of the database cached in memory, as that minimizes trips to the disk.
The Best Practices Analyzer still has a rule for this I think, where it will complain if the database and logs are not on separate drives, but the advice is still "meh" for the reasons I listed above.