If you've managed an Exchange server, you've probably heard someone say that the enterprise edition of Exchange supports up to four stores, or databases. In one sense, this is correct, but at the higher-level view, is not quite on target. In reality, Exchange Enterprise supports up to four stores per storage group, or up to 20 stores per server.
Let's take a high-level look at how Exchange stores data and make some sense of the limitations inherent in the different versions of Exchange server.
First of all, notice the use of two terms: storage group and store. A storage group contains at least one mailbox store. The individual stores inside the storage group contain mailboxes. The storage group can also contain a public folder store which, as you might expect, houses public folder information. Storage groups also house transaction logs for the stores/databases contained within.
Why might you want to create additional storage groups and stores? Well, under Exchange 2000, you don't want to create additional storage groups unless you have to. That is, if you hit the limit of four stores and need more, only then should you create a second storage group. This is due to the real potential for memory fragmentation and memory utilization inefficiency inherent in Exchange 2000's storage group model. The short answer to my question: In Exchange 2000, only create additional storage groups if you need more stores. Exchange 2000 SP3 does correct some of these issues, though.
Exchange 2003, on the other hand, makes some fundamental changes that make the use of multiple storage groups much more practical. You can use the single storage group mechanism and just create new groups as needed to support additional message stores. However, using a single storage group in Exchange 2003 can have negative disk performance consequences. Further, multiple storage groups can help to simplify and speed up the backup/restore process since transaction logs are not shared between storage groups (only between stores).
Additional stores are fairly common in organizations that need, for example, different rules for different sets of users. For example, you may have one set of users for which you want to limit their total mailbox size. For other users, you may want to provide an unlimited mailbox size. One easy way to accomplish this is to use separate mailbox stores and place each user's mailbox into the appropriate store.
So, for each version of Exchange, how many individual stores can you have per server?
- Exchange 2000/2003 Standard: One storage group with one mailbox store and one public folder store.
- Exchange 2000/2003 Enterprise: Up to four storage groups each with up to five stores for a total possible count of twenty stores per server.
Don't forget that Exchange 2003 also includes the Recovery Storage Group. This is not counted in the limits above and does not need to be. For example, under the enterprise version of Exchange 2003, you can have up to four storage groups plus the Recovery Storage Group.