The only favorable argument I've ever heard for Exchange's single instance store is disk space savings... MS proponents often claim (without data) that this is some kind of huge competitive advantage.  While I've tackled the subject generally in places like my Boss Loves Microsoft presentation, nothing's as good as real-world feedback.  Jack Dausman takes a look at this issue, in part based on an actual migration:

The Lower CPU usage I can explain in terms of the code rewrite for Domino 7, but using less disk space was completely unexpected. After all, I have always heard that the single-store architecture of Exchange provided the benefit of reducing disk resources (and, Domino has a single-store option as well for the same reasons).
Jack then uses the context of an article in Storage Magazine to find out why Domino is actually more efficient at disk usage than Exchange (go to his site to read the quotes from the article itself):
we simply run the compactor task to recover the data released from archiving. Doesn't Exchange have a compact task? Yes and no. For Exchange, the mail service has to be taken off-line for compaction.
Other reasons I think the Exchange guys are missing the point on single instance storage:

- Typically, Exchange doesn't scale to as many users per server as Domino, so there are more overall servers to deliver the message to
- Exchange servers have "storage groups" which means that messages are still written more than one time per server.  It's really the worst of both worlds -- a shared database that affects multiple people when there are problems, files that need huge amounts of memory to manage their I/O, and lots of contention on those files.
- While everyone loves to cite the example of the 10MB message sent to 500 people, these messages are the exception, rather than the rule, in any mail system..  The majority of e-mail messages are sent to one or two people, who may or may not be on the same mail database.  Have you ever looked at the tools MS provides to decide which storage group to assign a user to?  Oh wait, there aren't any.

Link: Jack Dausman: Exchange Storage Is Not So Neat and Tidy >

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