Loosing Single Instance Storage When Upgrading

This article applies to : Exchange 2000, Exchange 5.5

Originally Published : 1st December 2002

Last Updated :

There is some confusing information about whether or not single instance storage is retained when upgrading from Exchange 5.5 to Exchange 2000. This is enhanced with many vendors of migration products exaggerating the effects of single instance storage to promote their products. On the other hand, there is some truth to their claims.

Let's look at how single instance storage works in it's basic form. Server A, has 4 Users. User A sends an email to all the mailboxes on the server. As per standard Exchange Single Instance Storage practices, there is only one message stored with pointers to each mailbox. If we move User A to Server B, the single instance on that message has been broken (this is where the confusion starts). See the example on the left hand side of the screen :-

So now there is two copies of the same message. But when we move User B over, another copy of the message is not moved. This is because Exchange Server assigns an ID to every email. This is how it determines that the email is unique, and also determines whether the message is a duplicate. As the new message comes in, the other Exchange Server assigns it's ID to the message, and determines that there is already a copy of this message and therefore assigns a pointer to User B. Essentially, single instance storage is rebuilt.

To better understand we should look how Exchange stores a message within it's database and how it relates throughout the store.

The MsgFld table (Message Field) connects a Folder to it's messages. The first time a message is created in a folder, a MsgFld table is created. 

There is only one Messages table in an Exchange Database. This table contains all instances of data for the message as well as link to the Mailbox and Attachments tables. The message descriptions can be shared between the Mailbox and Attachment tables.

Like the Messages table, there is only one Attachment table in each Exchange database. This contains all instances of data for the attachment.

And similarly the Messages table the unique in each Exchange database. This table contains all instances of the data for the mailbox. Understanding how things relate to each other helps to understand the limitations of Single Instance Storage between databases, even if they are databases within a single Storage Group (Exchange 2000).

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