In the earliest of Exchange Server versions, the absolute maximum size of a mailbox database was 16 GB. As the versions increased and the years passed, this limit was retained in Standard Edition, but was removed in Enterprise Edition.
With Exchange Server 2003 service pack 2, the Standard Edition limit was raised to 75 GB (although this took a registry modification). With the release of Exchange Server 2007, Standard Edition has no limit on mailbox store size. And, in fact, the Standard Edition can have up to five mailbox stores and five storage groups (which greatly simplifies a number of disaster recovery scenarios - with this feature improvement it is extremely rare to ever need to build a recovery server any more).
But what does that REALLY mean?
Just because you no longer have any artificial limits on the size of your mailbox store, does that mean it is OK to allow the store to grow arbitrarily large?
The answer, as you might have already surmised, is a definite NO.
You, as an Exchange administrator (or someone who wears that hat among many other responsibilities), need to plan for reasonably sized Exchange mailbox stores. What is reasonable?
To answer it, I’ll need you to answer another question first: how much downtime can your business tolerate, while you restore an Exchange database from backup? Or, to put it another way, what is your SLA (Service Level Agreement) with your user community?
If this isn’t a conversation that you’ve ever had with your management - you probably should. Mistakes, errors, hard disk failures, etc. are simply a part of life in the computing industry. No person and no piece of hardware are perfect.
In general, if you have practiced database restores with Exchange Server, you can anticipate that a database restore is going to take about 20% longer than a database backup. You also need to assume that it will take you some time to conclude that a database restore is necessary and get your head around exactly the procedure and process you will follow in order to do the restore.
So, to pick a number out of the air, let’s assume that your Exchange mailbox store is currently at 40 GB and you do a full system backup at the same time you back up your Exchange server. Your backup takes two hours, to tape, and you figure that about half of that is from backing up Exchange.
You double the size of your message store, to 80 GB. Not too bad, you only add an hour to your backup. Now it’s taking three hours.
You double the size of your message store, to 160 GB. Ouch. Now your backup is taking at least five hours and maybe longer - and do you have to use multiple tapes now? What does that do to your cost-of-backup and your tape retention schedule?
And in each one of these cases, just assume that a restore will, at best, take only an hour more than the backup. So, with a message store of 40 GB you are down for three hours. With a message store of 160 GB, you are down for six.
Your best bet? Keep the store to a reasonable size.
Microsoft recommends around 50 GB. If you need to go larger, add another store. And when you back up, back up each store in parallel, so they get done more quickly.
Oh - use reliable hardware. Do you know what the best kind of restore is? It is the one you don’t have to do!