How Big Is "Larger"?

As almost any parent knows, young children have a knack for asking difficult questions. One of my favorite question types is the "relational" question, exemplified by queries such as "How long is 'a while'?" or "How much is 'some'?" Because Microsoft's been touting the ability of Exchange Server 2007 to handle "larger" mailboxes than its predecessors, the question "How big is 'larger'?" has started to pop up more often.

First, a bit of context: Exchange doesn't have a hard limit on the size of individual mailboxes, and it never has. (I dare say it never will, but don't take that as a promise until you hear it from Microsoft.) Exchange does have limits on the size of mailbox databases, which effectively caps the size of individual mailboxes. However, average mailbox size is more often limited by other factors, notably the number of disk I/O operations per second (IOPS) available for mailbox service, the total amount of disk space available, and the number of mailboxes that can be managed on a single server.

These factors are all interrelated, and they all tie in to backup and recovery speeds. Say you want to put 2000 mailboxes on a server. You can back up 20GB per hour on that server, and your service level agreement calls for a maximum of 6 hours of downtime for restores. Because restores take roughly twice as long as backups, your server mailboxes should be limited to about 30MB each:

(3 hours x 20GB per hour) / 2000 mailboxes = 30MB per mailbox

Of course, that's with current disk sizes; in the days when a 20GB disk was big stuff, you might have settled on smaller mailboxes or fewer mailboxes per server.

Now, 250GB and larger disks are becoming commonplace in new server installations. Exchange 2007 promises a large-scale reduction in the number of IOPS required per mailbox, thanks to its 64-bit-powered ability to use large amounts of RAM for the Extensible Storage Engine cache. Microsoft says you can now use much larger mailboxes without a performance penalty. But should you?

Maybe not. Allowing mailboxes to grow without limit can be a bad decision for several reasons:

I can already hear teeth clenching among the legions of administrators and users who hate mailbox quotas. It's true that you shouldn't impose arbitrary quotas—if you have a business requirement for large mailboxes, feel free to let them grow. However, just because you can support larger mailboxes on your existing hardware doesn't mean that you must, or even should, until you thoroughly understand the impact on compliance, productivity, and performance.

Many of the technical obstacles that limited the use of large mailboxes in older versions of Exchange, such as slow full-text search and I/O congestion, have been eliminated from Exchange 2007. If those technical limits were getting in the way of your business processes, you'll be able to move forward with allowing larger mailboxes. Just be sure you know what you're getting into.