I'm migrating from Exchange 2003 to Exchange 2010. There's a single Exchange 2003 server which isn't a domain controller, it's only running Exchange 2003. It has 100 mailboxes. The 2003 mailbox database folder size is around 22 GB.

I used the Exchange Storage Calculator and entered 150 mailboxes and a few other values and it suggested creating 4 mailbox databases. I've read several places which suggest using multiple mailbox databases but don't really explain why.

  1. Does it matter how many mailbox databases I use? Should I use 4 or more or less? Why?

  2. When I performed a test mail migration from 2003 > 2010 all of the migrated mailboxes end up in the first of the four mailbox databases I created i.e. MD01. Is that because there are only around 22 GB of files and there weren't enough files to be spread out among the 4 databases or do I need to do something special to perform a migration from 2003 > 4 mailbox databases in 2010 ?

Thanks in advance


Spreading out your mailboxes across databases is good to keep database sizes low. If anything goes seriously wrong and you have to run offline utilities to get things back, you will thank yourself for keeping smaller databases. Users do not like being without email for the 8 hours it takes to recover a 25GB database.

As for targeted moves, there is a way to move users into specific databases. We did this when we did the 2003 > 2007 move. I believe you need to use Exchange 2010 tools to do the migration, and you can pick the target database. We used a PowerShell script to perform the migrations, which may expose a few more controls to manage this.


If your organization is unlikely to grow, you could argue that you don't need more databases. However, scaliblity is a huge reason to split out your datastores. This would allow you to share the load between EX servers easily, especially as you grow. (Only having one EX server is a bad thing, btw. You should really push to buy the resources to make it a cluster.)

Ease of management is another reason. Everyone has bad users who don't know what the delete button is. Having multiple datastores, even a 'can behave themselves' and 'delete broken' prevents one runaway mailbox from breaking everyone's mail when the DB fills up.

There are plenty more best-practices regarding datastores to read up on, more reasons than I can state here. Search Microsoft for "Exchange 2010 Best Practices."


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