Microsoft believes that moving to a 64-bit platform can solve many of the longstanding problems in the Microsoft Exchange Server Information Store. More addressable memory will make fragmentation of virtual memory far less of a problem, so Exchange can support more storage groups (SGs) and databases. At the same time, the extra memory will let the Store cache much more data than it can today, so Exchange should generate far fewer I/O operations. If Exchange can cache every user's Inbox, the rules that typically process messages arriving into the Inbox, and the 50 most-recent messages, the number of I/O operations will be reduced significantly because the Store can provide this data from memory instead of constantly going to disk, even on large servers. One calculation from the Microsoft Exchange Engineering group shows that the current 32-bit Jet database cache can store 250KB per user on a server that supports 4000 mailboxes. By comparison, on a 64-bit server equipped with 16GB of RAM, the expanded Jet cache can hold 3.5MB per user for the same number of mailboxes. That's a 14-fold increase in the amount of data that the Store can access from memory rather than having to go to disk. Note that the extended cache doesn't automatically mean that the Store will generate 14 times fewer I/Os, but trading expensive storage I/O for relatively cheap memory access is obviously a good idea.
Apart from the increased cache, Microsoft is changing the Jet page size in Exchange 12 from 4KB to 8KB so that the Store can hold more message bodies in a single page. (The message body is the content that you type into the cover note together with autosignatures and other data and doesn't include any attachments that you send along with the final message.) Microsoft believes that this increase will let the Store hold more than 50 percent of all messages in one page with a subsequent decrease in the number of read and write operations that the Store generates for each message.
Testing is going to be necessary to get real-life production data to verify-exactly what effect 64-bit Exchange will have on I/O operations per second (IOPS). Educated feedback from those involved with Microsoft engineering puts the figure at between 0.3 and 0.25 IOPS per user. This data was gathered from some of Microsoft's systems, which had been operating at 1 IOPS per user on a 32-bit platform. This type of reduction theoretically can reduce your storage costs, although experience demonstrates that such savings are difficult to achieve because you aren't likely to jettison all your existing storage and replace it with lower-cost alternatives. Instead, you'll probably keep the same storage infrastructure, but if the I/O demand drops, you can use the storage to host more servers or to handle the increased workload of new applications (e.g., unified messaging). Exchange 12's real cost savings will appear through consolidation and extended storage lifetime (rather than the installation of cheaper storage solutions).