Up to this point, our discussion on the Store has centered on the classic situation where every Exchange server operates a single Store composed of two databases the private store for mailboxes, and the public store for public folders. To a certain extent, hardware has now outstripped the capacity of Exchange to scale past a certain point using a single Store. Even the smallest server offered by major vendors is easily capable of supporting hundreds of clients, and if you upgrade the server with an extra CPU, more memory, and an enhanced disk I/O subsystem, the system will be able to support many thousands of clients. You can keep on adding CPUs, memory, and disk and even cluster servers together, but most experienced Exchange designers recommend that you don't place more than 3,000 mailboxes on a single Exchange 5.5 server (or cluster). This is a self-imposed limit, and one that appears to run counter to the results of LoadSim tests that vendors report for hardware. Some of the latest 4-way and 8-way servers seem to be able to support over 30,000 concurrent MAPI clients, so what's to stop anyone from building a production server to host a similar population?

The answer is simple: data. The Exchange 5.5 Store is divided into the public and private stores, each represented by a single database file called PRIV.EDB and PUB.EDB. The private store holds all the user mailboxes and tends to be larger than the public store, especially on very...