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Cached Mode

Cached Mode

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Cached Exchange Mode was introduced with Outlook 2003. When an Outlook account is configured to use Cached Exchange Mode, Outlook works from a local copy of a user's Exchange mailbox stored in an Offline Folder file (OST file) on the user's computer, along with the Offline Address Book (OAB). The cached mailbox and OAB are updated periodically from the Exchange server. At the same time, Outlook 2003 maintains an online connection to a remote copy of your mailbox in Exchange Server.

The time that is required to complete the initial synchronisation between Outlook 2003 and Exchange Server 2003 depends primarily on the size of the mailbox and on the speed of the connection to the Exchange Server 2003 computer.

Access to all data is not available until the initial synchronization is complete. Therefore, it is recommended that a fast connection is used when Cached Exchange Mode is started for the first time for each user.

After the initial synchronisation is complete, Outlook 2003 would keep the local copy up to date automatically. If a change was made to the data on the server, Outlook 2003 would be notified to synchronise the changes. Changes on the server may occur if a new message was received, or if another client made a change to existing data. If changes are made to the local data, Outlook 2003 synchronises those changes with the server automatically. This process occurs in real time and does not require user intervention.


Outlook 2003 Cached Exchange Mode offers the following benefits:

·         After messages have been cached locally, typical user operations do not cause interactions that block the server. Marking a message as read, replying, and editing require a small amount of data to be pushed up to the server to keep the mailboxes synchronised. However, the pushing of data occurs in the background. This behaviour causes much faster access to messages and to attachments, because work is done from the local copy instead of the server copy.

·         Cached Exchange Mode causes no loss of conventional functionality. New e-mail notifications, full Global Address List details, free/busy lookup, public folder access, and delegate support function as expected. However, this is true only when a network connection to an Exchange Server computer is present. 

·         Cached Exchange Mode provides intelligent use of bandwidth. This functionality is enabled by synchronising only headers on slow connections (connections that are slower than 128 kilobits per second [Kbps]). This functionality works only when a network connection is present. 

Additionally, Cached Exchange Mode offers administrators the following benefits:

·         Reduced server load. After messages are cached locally, re-opening the same message does not require server transactions. 

·         Reduced network load. After messages have been pulled over the network one time, subsequent access to those messages does not cause additional network traffic. Because messages are also compressed, there is an additional reduction on network load. 


When and who should run cache mode? It all depends on the cache size and performance of the local pc.


Effect of Online Mode Clients

Unlike Cached Exchange Mode clients, all Online Mode client operations occur against the database. As a result, read I/O operations will increase against the database. Therefore, the following guidelines have been established if the majority of clients will operate in Online Mode:

  • 250 MB Online Mode clients will increase database read operations by a factor of 1.5 when compared with Cached Exchange Mode clients. Below 250 MB, the impact is negligible.
  • As mailbox size doubles, the database read IOPS will also double (assuming equal item distribution between key folders remains the same).

The following graph illustrates IOPS based on mailbox size.

Database read IOPS increases as mailbox size increases


Read IOPs increase as Mailbox size increases

Testing has also shown that increasing the database cache beyond 5 MB per mailbox will not significantly reduce the database read I/O requirements. The following graph depicts 2-GB mailboxes using Online Mode clients and the effect increasing the cache beyond 5 MB has on reducing the database read I/O requirements.

Database read IOPS decreases cache size per mailbox increases


Read IOPs increase as Mailbox cache increases

As a result of this data, two recommendations can be made:

  • Deploy cached mode clients where appropriate. See the "Item Count per Folder" section below for more information.
  • Ensure that the I/O requirements are taken into consideration when designing the database storage.

For additional IOPS factors, such as third-party clients, see Optimizing Storage for Exchange Server 2003.




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