Single Instance Store in Exchange 2010

Microsoft Exchange Team Blog

In Exchange Server 2010, there is no more single instance storage (SIS). To help understand why SIS is gone, let's review a brief history of Exchange.

During the development of Exchange 4.0, we had two primary goals in mind, and SIS was borne out of these goals:

  1. Ensure that messages were delivered as fast and as efficient as possible.
  2. Reduce the amount of disk space required to store messages, as disk capacity was premium.

Exchange 4.0 (and, to a certain extent, Exchange 5.0 and Exchange 5.5) was really designed as a departmental solution. Back then, users were typically placed on an Exchange server based on their organization structure (often, the entire company was on the same server). Since there was only one mailbox database, we maximized our use of SIS for both message delivery (only store the body and attachments once) and space efficiency. The only time we created another copy within the store was when the user modified their individual instance.

For almost 19 years, the internal Exchange database table structure has remained relatively the same...

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