In my last article, “Mail Flow in Exchange Server 2007”, I introduced the concept of bifurcation. Bifurcation is a fun word to say, but it’s important in reducing bandwidth consumption and in making disk storage usage more efficient. In fact, support for delayed bifurcation is what leads to Single Instance Storage in an Exchange mailbox database.

In simple terms, bifurcation means that you take a single message and split it into multiple copies, preferably only when you are required to make the split. The process of delaying as long as you can in making that split is called delayed fan out and it is also commonly referred to as an Exchange 2007 feature.

All of these are related. Let’s discuss them and see how they apply.

Single Instance Storage (SIS) applies to special messages. The requirements for a message to use SIS are:

 1] Destination recipients are all in the same Active Directory site

2] Destination recipients are all on the same mailbox server

3] Destination recipients are all in the same mailbox database

If all of these three requirements are true, and there are multiple recipients for a message, then only a single copy of the message’s attachments is stored into the destination mailbox database.

Yes, I said a single copy of the attachments. In releases of Exchange prior to Exchange 2007, only a single copy of the entire message was stored (message body plus message attachment). However, that had a high disk cost (that is, it required more input-output operations than storing multiple copies of the message); and one of the major goals of Exchange 2007 was to reduce disk cost. Many of the tradeoffs involved in reaching that goal required using more disk space in order to reduce various I/O operations.

Attachments are accessed less often than message bodies, so they retain SIS. Also, note that Microsoft recommends against using SIS when planning for your disk usage requirement in Exchange Server.

Next, fan out describes when bifurcation occurs. In releases of Exchange prior to Exchange 2007, if the transport engine detected that the destination recipients were on separate Exchange servers, then bifurcation happened immediately, even if those recipient Exchange servers were side-by-side four routing groups away (that is, multiple copies of a message would cross the network). In Exchange 2007, bifurcation is delayed as long as possible, which leads to the feature known as delayed fan out.

If the Exchange servers for two recipients are in the same Active Directory site, but two separate servers, the bifurcation of the message does not occur until the message reaches a hub transport server located within the destination AD site. If two recipients are on the same Exchange server, but two separate mailbox databases, bifurcation does not occur until the message reaches the information store service on the destination mailbox server. And finally, if two recipients are on the same mailbox database, bifurcation of the message body happens when the message is stored into the database, bifurcation of any attachments does not occur.

Delayed fan out can result in significant bandwidth savings if many of your destination recipients are not in your local Active Directory site. SIS can reduce the storage used by attachments in your Exchange mailbox databases.

Bifurcation is a basic process of all mail transport engines. In this article, you have learned how it is used within Exchange.