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After an Exchange Server computer has been in service for a few months, you may notice that the Single Instance Storage Ratio is declining. The lower this number is, the less storage benefit you are receiving from single instance storage. If this number reaches 1, there is effectively no storage benefit being obtained from single instance storage.
This is normal behavior and should be expected to occur. The longer a database has been in service, the lower you should expect the ratio to be. The factors discussed in this article apply not only to Exchange Server but to any messaging system that implements single instance storage.
Single instance storage means that if the same message is sent to two users on the same server, that only one copy of the message is inserted in the database. Both users access the single instance of the message to read it. If this were the only message in the database, the single instance storage ratio for this database would be 2.
If the message were sent to four people, two on your own server and two on another, only a single copy of the message would have to travel to the other server to deliver the message to both users.
Thus, single instance storage ensures that only one copy of a message is needed on any given server, no matter how many users receive the message.
You may monitor your storage ratio in Performance Monitor using the MSExchangeIS Private and MSExchangeIS Public objects Single Instance Ratio counter.
There is a common misconception that the primary benefit of single instance storage is that it greatly reduces the storage space requirements for user data on a mail server. The truth is that its primary benefit is to greatly enhance delivery efficiency of messages sent to large distribution lists. Disk space savings from single instance storage are transient and drop off very quickly over time.
For Example: Suppose a message is sent to 500 mailboxes on a single server. This gives you a huge initial ratio of 500. Within a day or so, 80 percent of the recipients are likely to have deleted the message, plunging your ratio to 100. Over the next week, probably 98 percent will delete it, cutting your ratio to 10. Making the drop-off curve even steeper is the general rule that the larger the distribution list, the shorter the life of the message.
The most important benefit of single instance storage in sending this message was that it got delivered with approximately one-five hundredth of the work that would otherwise have been necessary--one copy was generated instead of 500. Between servers, single instance storage greatly reduces the network bandwidth required to transmit a message with a large distribution list.
Even a "mature" server is likely to have a storage ratio slightly greater than 1, but you should not count on single instance storage at all when doing capacity planning. In some cases, the ratio may even drop slightly below 1 because of miscellaneous database overhead.
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