When Microsoft first shipped Exchange Server 4.0 in 1996, the company touted the benefits of single-instance storage. In fact, later versions of competing products, such as Lotus Domino, also included this feature. So what's happened to single-instance storage in Exchange, and what are the related implications of migrating from Exchange 5.5 to Exchange 2000 Server?
Single-instance storage in Exchange reduces storage overhead by storing shared messages and attachments only once per Store. In theory, single-instance storage helps reduce the size of the Store and offers benefits for disaster recovery and Store management, especially in small and medium-sized deployments. For large-scale deployments, single-instance storage is more difficult to achieve because the required grouping of workgroup users (who usually have a high incidence of common mail data and attachments) on the same servers across a large enterprise is difficult for administrators to manage. Microsoft realized (through customer feedback) that high single-instance storage ratios (i.e., how many mailboxes reference messages in a Store) are difficult to achieve for most Exchange deployments. This fact, along with the inclusion of single-instance storage technology in competitive products, has caused Microsoft to de-emphasize the importance of the technology in Exchange.
How does single-instance storage affect your migration from Exchange 5.5 to Exchange 2000? This question is interesting because of Exchange 2000's new storage paradigm. In previous Exchange versions, each server hosted one Store, so you could manage single-instance storage on a per-server basis. However, Exchange 2000 supports up to 20 Stores per server, so single-instance storage becomes even more challenging to manage. Remember that single-instance storage applies within only one Store. When you partition users among several Stores, single-instance storage breaks down.
The migration method is a key factor in maintaining single-instance storage when you move mailbox data from Exchange 5.5 to Exchange 2000 servers because the Store uses the MESSAGE-ID of each message to manage and track single-instance storage. If your migration tool doesn't maintain Exchange's protected storage environment (leaving MESSAGE-IDs intact and in context), the migration process won't maintain single-instance storage. Tools such as Exmerge move messages out of protected storage, whereas tools such as Move Mailbox in the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in maintain the protected storage environment. If you're planning to migrate from Exchange 5.5 to Exchange 2000 and want to maintain single-instance storage, I recommend you test your migration tools in a lab environment to determine how they affect single-instance storage during mailbox migration.
Microsoft no longer hypes single-instance storage in Exchange Server, but the feature is still around and will affect how you manage and maintain your Exchange data. Take time to consider and weigh single-instance storage technology benefits in light of the management challenges you'll face trying to maintain it.