How does Exchange Single Instancing work in Archive Manager?
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How Exchange Server handles single instance?
Single instancing in exchange is only effective within a private store. According the developers of Exchange, it does not, and will not in the foreseeable future, support single instancing across private stores.
Example 1:
If a message is sent, with attachment to say 20 users on the same store, the attachment is stored once because of single instancing. Once the message is modified, (e.g., it is forwarded, or open and modified in the mailbox - say for e.g. change the subject), the attachment is stored again in Exchange. So if a message with an attachment is sent to 20 people on the same store, then 5 of those people forwards the same message, 6 copies of the attachment stored in Exchange.
Example 2:
If the same attachment is sent to 20 different people on the same store as a separate message, the attachment is single instanced for those 20 people, but its stored as a separate record to the original attachment - so there are now 7 copies of the same attachment in that exchange store (included the 6 from example 1).
How Archive Manager handles single instance?
Archive Manager doesn't look at the message as a whole when single instancing. Archive Manager breaks it down into the body and attachments.
So going back to example 1 in Exchange, with 20 recipients + 1 sender and then the five people forwarding the message, how does it look like in Archive Manager?
The original message would be stored once, and it would appear in the sent items and the 20 recipient's inbox. The attachment would be stored once. The 5 forwarded messages would be stored as 5 different messages (as they would be from different senders), but the attachment would not be stored again. So, instantly, instead of storing 6 attachments as Exchange has, we still have the one. Now if in point 3, the same attachment sent to 20 recipients, its not stored again, so still one attachment.
In addition, Archive Manager does not know the difference between MOVE or COPY in Exchange. If a Message is copied to another folder, and Exchange Store Manager crawls that folder, it will link that item to the last folder it scans instead of being linked to both.
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