In year may 1999 Tech-Ed Microsoft represented declared, Exchange is the first messaging system which is using SIS. It is up to you believe Microsoft product manager or not, I am just a messenger. What is this SIS? It is shared message storage if you would think so, Exchange stores copy of message and creates pointer to multiple mailboxes within the same mail store. If a message is sent to one recipient, and if the message is copied to 20 other recipients who reside in the same mailbox store, Exchange Server maintains only one copy of the message in its database. Exchange Server then creates pointers
These pointers link both the original recipient and the 20 additional recipients to the original message. If the original recipient and the 20 additional recipients are moved to another mailbox store, only one copy of the message is maintained in the new mailbox store.
As you can imagine the benefits clearly listed below
- Reduce I/O positive impact
- Less work on Exchange, more power for non redundant work
- Without it, Exchange may go crazy and losing all its resources, imagine a mail 10Meg send to 1000 people on the same mail store.
- Reduced storage improvements ( less data to store obviously)
- Any other similar effect you can think of.
Think all the garbage you get when you receive your newspaper, free coupons etc. Imagine in your neighborhood 10 homes receive all these garages, or instead there is a shared mailbox, postman puts only one copy of each garbage collection so that people can read if they want it.
In Exchange 5.5, there is one database on the server. Mail sent to multiple mailboxes on that server is only stored once, with pointers delivered to each recipient. In Exchange 2000 and Exchange 2003, you can have up to 20 databases, where each database could have one copy of the message should recipients reside on each database. Each additional database adds an additional 2 percent to the database IOPS. How well Exchange utilizes single instance storage depends on the percentage of time messages are sent to recipients on the same database, and the average message size. Larger messages have more benefit with single instance storage.
Lastly I want to mention about Brick level backup. Why everyone don't recommend brick level backup? Lest start with Microsoft, the company who invented Exchange server application does not recommend brick level backup, due to several reasons. Introduction of RSG in exchange 2003, there really is not logic having brick level backups; it is such a waste of time and resources. I happen to have experience recently , one of your helpdesk administrators could not find out the reason, why he was backing three times more data than, his actual database sizes when he performs brick level backup. The reason easily can be discovered with understanding the SIS and how it works.