When should I use multiple storage groups (SGs) and multiple databases in Microsoft Exchange Server 2003?

Each SG has its own transaction logs, so if you use multiple SGs, each with its transaction logs set on different disks, you should see performance gains.

Some mail accounts might require a lower level of recoverability. For those accounts, you don't need to keep transaction logs, which means you can enable circular logging. (To learn more about circular logging, see the FAQ "How do I enable circular logging for Active Directory?" at http://www.windowsitpro.com/Article/ArticleID/13403/13403.html .) You can place accounts that have different recoverability requirements into separate SGs, then enable circular logging on the SGs containing the accounts that don't have high recoverability requirements.

You can set certain administrative options at SG levels.

You can restore only one database at a time per SG. Consequently, spreading databases over multiple SGs enables more concurrent database restores.

You can place each database on a different physical disk, which will likely improve performance.

You can set database-level quota policies, so that by grouping users into different databases, you can assign different policies (e.g., mailbox quotas) to particular user groups.

By separating users into multiple databases, you minimize the scope of any database corruption.

In the event of a database restore, you must restore the data in only one database, which enables faster recoverability.

By dividing user accounts over multiple databases, you can prioritize the database-restoration order in the event of a total disaster. For example, if all managers are in one database, you could restore that database first. (Of course, when you put all managers in one database, if a database becomes corrupt, it's bound to be that one!)

Multiple databases let you keep database size to a manageable level (typically less than 40GB).