There is a new article on TechNet called “Capacity Planning for Active Directory Domain Services” which outlines key items for planning Active Directory Domain Controller capacity (Windows Server 2008 and newer).
Here are the highlights:
- Plan for the peak busy period of the day. It is recommended to look at this in either 30 minute or hour intervals. Anything greater may hide the actual peaks and anything less may be distorted by “transient spikes.”
- CPU Sizing: 1 modern physical core per 1,000 users (authentication).
- RAM Sizing: 2-4 GB for OS + size of NTDS.dit + size of SYSVOL.
Also, don’t forget to perform core calculations to ensure appropriate capacity for Exchange (8:1 ratio for Exchange cores to GC cores) and other applications.
I typically recommend that at least 1 DC is on physical hardware in each domain – preferably 1 physical DC per domain in each datacenter. DCs should be spread across virtual hosts as much as possible to provide redundancy and ensure there’s DC capacity for virtual clients and application servers.