Single Instance Support – Exchange Server 2003
Exchange Server 2003 fully supports single instance messaging.
If a single message is addressed to multiple recipients and those recipients are located in the same database, only one message is sent to the database. The recipients receive pointers to the single message.
If message recipients are located on multiple databases in a storage group, a single copy of the message is delivered to each database where recipients are present. Additionally, multiple copies of the message are held in the transaction log set for the storage group.
There are many theories about how to make the best use of single instance support. Some people prefer to locate whole departments of users in the same database, therefore potentially saving disk space; others do not like this approach as a database failure can make an entire department become unavailable.
Single instance storage is designed to improve the delivery performance of the server when delivering to many people, and not necessarily to save space on the disk.
Single instance is also retained if you move mailboxes between Exchange databases, storage groups, and servers.
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