Client Configuration

A variety of clients are able to connect to Exchange Server 2010. Other than receiving e-mail, the client's activity is the largest driving factor on how a mailbox server performs. A number of things should be done to improve performance and protect the mailbox services.

In Office Outlook 2003, a new feature was introduced called Cached Exchange Mode. This was different than offline mode, available in previous versions. Offline mode provided a way for portable computers to synchronize data and then be able to work offline. Cached Exchange Mode solved the same problem offline mode did; however, it also reduced the bandwidth requirements between the client and Exchange server for clients that were connected to the server. This allows users access to the local copy of the messages stored in a OST file very quickly and synchronize the changes between the server and the client. This reduced the I/O and bandwidth (after the initial synchronization) by up to 20 percent, a great way to deploy Outlook in most situations.

Note: For Outlook 2007 clients, ensure that the February 2009 cumulative update for Outlook 2007 Service Pack 1 or later has been deployed to provide the best performance.

Cached Exchange Mode does not work well in just a couple of situations. If a mailbox is 100 MB, that data is stored locally and then synchronized as changes are made. As the mailbox size grows, more resources are needed on the client to cache and synchronize the data. On older clients that do not have enough drive space to index and manage the amount of e-mail in a mailbox, large mailboxes may perform poorly, even if the mailbox server is not performing poorly. It could be that additional resources should be planned for clients with large mailboxes that will use Cached Exchange Mode. When you deploy client computers that are running Outlook in Cached Exchange Mode, consider the following performance improvement guidelines:

• Ensure that the February 2009 cumulative update for Outlook 2007 Service Pack 1 or later has been deployed. This update included a new OST schema which reduces I/O requirements. Outlook 2010 also includes these performance improvements. For more information about the update for Outlook 2007, read the Knowledge Base article at http://support.microsoft.com/kb/961752.
• For mailboxes up to 5 GB, most client computers capable of running Outlook 2007 or higher should perform well.
• For mailboxes larger than 5 GB and up to 10 GB, additional hardware may be required. This will often be limited by disk I/O and the amount of memory. Using a RAID10, solid- state disks, and faster traditional hard drives for storage of the OST file will greatly improve performance.
• For mailboxes with more than 10 GB, additional hardware and memory will be needed. Archived data should be moved to an online archive and is thus exempt from being cached locally.
• For mailboxes that have more than 100,000 items in a single folder, views other than Arranged By: Date may be slower.
• Limit the amount of data that is available in the cache. By using the filter options available on each folder, the amount of data that is cached on the local computer can be limited. A typical example of this might be to only cache data that is less than a year old.

Up until Outlook 2010, one of the other places where Cached Exchange Mode is not supported is when Outlook is hosted on Remote Desktop Services (RDS), formerly known as Terminal Services. In RDS multiple servers will be logged on to a single server accessing Outlook, sending and receiving e-mail. The best practice in deploying RDS for high availability is to use roaming profiles and to not store the user's data on each RDS server in the farm. RDS is deployed in the datacenter so that it can be managed by the IT staff. The users that access the RDS may be located in other locations or have limited bandwidth, which is a good reason to have Outlook run in the datacenter. The RDS servers are typically located near the Exchange servers; RDS users don't usually store data on each server. Therefore, if you're using an OST to synchronize data-especially if each time the user logged in Outlook had to cache all of the mailbox data-it is understandable why using Cached Exchange Mode is not recommended nor is it supported in that configuration.

Using Cached Exchange Mode performance can be limited by the local hardware. When using Online Exchange Mode, the client requests information directly from the server. In Exchange Server 2010 using Outlook in Online Mode or Outlook Web App, the guideline is to limit the number of items in each folder to fewer than 100,000 provided there are no third-party applications installed that index e-mail content. This is a huge jump from Exchange 2007 SP2 where the guideline was a maximum of 20,000 items for Inbox and Sent folders and 5,000 for Calendar and Contacts folders.

Many Outlook add-ins and desktop search software will add additional load to the client and the mailbox server if they perform indexing or data mining. Providing end users with guidance on software that they should avoid or using permissions and policy to restrict users from installing these applications helps to reduce the likelihood that these applications can negatively impact performance.