I thought I’d blogged about this before, but apparently not. I’ve gotten a few questions at roadshow events about how the Exchange Unified Messaging server role scales. Now, it’s not entirely fair to ask scalability questions about products that are still in beta because the answers are almost guaranteed to change (and hopefully for the better). However, in discussions with Microsoft’s Michael Khalili, I understand that the current guideline is that a single server should be able to handle 80-100 concurrent calls (the direction doesn’t matter, whether inbound or outbound). If you co-locate the UM role on another server, you may be able to handle fewer calls, but as with so many other scalability questions, the ultimate answer is “it depends”.
Observationally, I’ve been able to easily handle 4-5 concurrent UM calls on a 32-bit VM running as a UM / mailbox / CAS / hub transport server. I’m sure once Microsoft IT rolls out Exchange UM across the company they’ll be publishing one of their nifty “IT Showcase” white papers that describes in detail what their architecture looks like.
Update: forgot to mention Michael Wilson’s excellent post on the number of users you can put onto a UM server.
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