Question regarding the rule of thumb regarding Exchange proc cores ratio to GC proc cores

Question

Hi all,
I am studying towards my MSCE Messaging for Exchange 2013 and came across on CBT nuggets a statement regarding a rule of thumb when it comes to processor cores on Exchange 2013 server to GC
so Greg Shields from CBT nuggets says that the ratio should be
4:1 on x86 domain controllers
8:1 on x64 domain controllers.
So is he talking about 1 mailbox server role with 8 processor cores to 1 processor core on a x64 domain controller? what if there is already a heap of servers using that GC? In our datacentre we have 2 x64 domain controllers but also a whole range of servers
on that server network.
Also what if you have 2 mailbox server roles? are we then talking 16 : 2 ?
Hope someone can help clarify this.
Regards
ronnie.jorgensen systems engineer
My blog

Answer 1

Does this help explain it better?
http://blogs.technet.com/b/exchange/archive/2013/05/06/ask-the-perf-guy-sizing-exchange-2013-deployments.aspx
Active Directory sizing remains the same as it was for Exchange 2010. As we gain more experience with production deployments we may adjust this in the future. For Exchange 2013, we recommend deploying a ratio of 1 Active Directory global catalog processor
core for every 8 Mailbox role processor cores handling active load, assuming 64-bit global catalog servers:
If we revisit our example scenario, we can easily calculate the required number of GC cores required.
Assuming that my Active Directory GCs are also deployed on the same server hardware configuration as my CAS & Mailbox role servers in the example scenario with 12 processor cores, then my GC server count would be:
In order to sustain double failures, we would need to add 2 more GCs to this calculation, which would take us to 7 GC servers for the deployment.
As a best practice, we recommend sizing memory on the global catalog servers such that the entire NTDS.DIT database file can be contained in RAM. This will provide optimal query performance and a much better end-user experience for Exchange workloads.
and of course use the latest calculator version to determine your required specs:
http://blogs.technet.com/b/exchange/archive/2013/05/14/released-exchange-2013-server-role-requirements-calculator.aspx
When you calculate required GC cores, the ratio 1:8 applies to the cores required to support active mailboxes ("Mailbox role processor cores handling active load") and not to all cores physically available on Exchange server.
So, for example, if you have 32 total cores on an Exchange server but only 16 of them are actually needed to support active mailboxes (as determined by the calculator), you will need only 16/8=2 GC cores to support this Exchange server's AD queries.
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