THE OFFICIAL BLOG OF THE SBS "DIVA"

From the AD perspective.....3GB is goodness.
When you toss 3GB on a box, you open up AD to another GB of cache it can use. Basically, we cache pages from the database (pages are 8KB each in AD). W/o /3GB on 2k3, you can cache ~1.5GB of your database. With /GB, you get an extra gig.
Similar logic can be applied to Exchange (numbers vary though).
It comes with the price of less kernel mode memory (things like fewer PTEs) but for boxes that are just DCs, that's ok.
For Exchange servers, there is Exchange specific logic. ExBPA will tell you the values, but I believe they recommend /GB and /userva=3030, but double check me on that, I'm not an Exchange guy.

I would point out...for AD and Exchange, if there is memory pressure, the benefit of /3GB might be questionable. If there is no memory for the ESE cache to use, well, the value of saying "you can use more" might not be there. :) That'd be fairly circumstantial I suppose, so you'd need to look at your own box and how many other apps are on the box.

And for SBS, I have no idea. :)

Does that mean that we shouldn't be doing the following registry edit as well?

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager
Value name: HeapDeCommitFreeBlockThreshold Value type: REG_DWORD Value data: 0x00040000

The primary benefit of /3gb switch (or the HeapDeCommitFreeBlockThreahold setting) is related to intense memory pressure in the user memory space. This is something you'd be very hardpressed to see in SBS since it is almost singularly related to server scale... it's the 4 fully-loaded storage groups with thousands-upon-thousands of mailbox users connected that leads to this scenario. Definitely not sometihng you'll find in SBS.

So, can you do it on SBS? Absolutely! Should you do it on SBS? Less clear. You might see a little-bit better performance if you've got >1gb of physical memory or you manually increase the ESE buffers and have /3gb switch set, but that'd be about it, I think.

I wouldn't expect a tremendous perf gain for SBS using /3gb for several reasons.

1. The AD DIT is bound to be tiny, we are talking small business, isn't that like 25 users? I wouldn't be expecting 500MB DITs let alone multiGB DITs.

2. Exchange might like the added memory but as indicated by Eric, this is impacted by memory pressure and you have quite a few things loaded on SBS causing this pressure. It isn't like application specific installs where you segregate apps across multiple servers. SBS is the kitchen sink and everything else on one box.

Those two KB's imply that the entire SBS concept of Exchange running on a domain controller isn't advised:

"We recommend having dedicated Active Directory Domain Controllers or Global Catalog servers."

Why recommend a dedicated server for a domain controller? Just to sell more copies of Windows Server 2003, or for actual (measurable??) stability concerns?

They can't have it both ways... "We recommend seperate Exchange servers and domain controllers, but we also offer an SBS package that doesn't follow these recommendations. Why? Because if we can't fool you with stability and security fears into buying several copies of Server 2003 , we'll still get at least one purchase out of you. Thanks for shopping at Microsoft."