Executive Summary:
Microsoft Office Outlook uses the Offline Folder file (OST) to synchronize files for offline access but increasing mailbox sizes have led to sluggish performance because OSTs weren't designed to handle such large files. You can compact OSTs, run Scanost.exe, or rebuild OSTs to improve performance; however, Microsoft Office 2007 SP2 addresses the performance problem and provides quicker responses on large mailboxes. An additional fix lets Outlook shut down faster, which helps avoid OST corruption. |
Microsoft Exchange Server administrators don't typically get very excited about a Microsoft Office system service pack, perhaps because it's usually someone else that takes care of updating software on PCs. However, with Office 2007 SP2, Exchange admins—and end users—might have reason to celebrate. Office 2007 SP2 includes fixes for a major performance problem in Microsoft Office Outlook 2007 that has affected Microsoft Exchange Server for years. Even better, because the results of the performance work were so good, Microsoft decided to release an update for Outlook 2007 SP1 in the February 2009 Cumulative Update that you can apply immediately without waiting to roll out the full SP2. Let's take a look at why Outlook performance has suffered on large mailboxes and examine the improvements that Microsoft has made.
The Zen of the OST
MAPI-based Exchange Server clients have always supported the Offline Folder file (OST). Back in the mists of time, early versions of Outlook and the original Microsoft Exchange Viewer used OSTs to synchronize folders for offline access on an on-demand basis. Important folders such as the Inbox were synchronized automatically, but if you wanted to use other folders offline, you had to configure the client to synchronize them whenever the it connected to Exchange. Folders in the OST are replicas of the online master folders in user mailboxes; the synchronization process keeps the two copies aligned. Unlike PSTs, which can be opened by any MAPI client, an OST is always linked to a specific user mailbox and can be opened only by a client that can connect to that mailbox.
Simple synchronization was sufficient in the era of dial-up networking when a 56Kbps connection was something to celebrate. A major change occurred when Microsoft introduced Outlook 2003 alongside Exchange Server 2003 and we entered the era of Cached Exchange Mode. Exchange 2003 introduced features such as drizzle-mode synchronization and a bunch of networking improvements that Outlook exploited to capture a complete copy of user mailboxes in the OST. This change was great from a user perspective because you had a complete copy of your mailbox to work on even when the network wasn't available.
The introduction of Cached Exchange Mode was popular with administrators because it facilitated server consolidation. Previously, clients had to connect to a local Exchange server when suitable communications weren't available to allow online access to servers in centralized data centers. With Cached Exchange Mode, those same clients could connect over extended links and synchronize behind the scenes while users worked with data in the replica folders in the OST. Cached Exchange Mode also isolated clients from temporary network outages, and the Outlook 2003/Exchange 2003 combination was more effective in using network bandwidth so more clients could work across the same link.
Maintaining OST Performance
Cached Exchange Mode remains a tremendously important aspect of Exchange design today. However, the OST file structure wasn't designed to handle the size of mailboxes that are now common. OSTs cope splendidly when only select folders are synchronized from small mailboxes; they're much less effective when you synchronize all the folders from multigigabyte mailboxes. My personal experience is that performance erodes after an OST grows to more than 1.5GB, more than 5,000 items in a folder, or more than 20,000 items in the OST, but this can vary according to the speed of your PC's hard disk (as measured in its ability to transfer data; PCs equipped with solid-state disks are typically the fastest, those with 7,200 RPM standard disks are the next quickest). You know an OST is having problems if the hard disk indicator of your PC stays constantly lit when Outlook opens a new folder or performs other operations that force Outlook to access data in the OST.
An OST is typically between 10 percent and 15 percent larger than its source mailbox because of the way that data is organized in the file and the internal structures that index the data. You can't restore performance simply by deleting items from your online mailbox to reduce the size of the OST and the number of items it holds because Outlook has to perform some processing to compact the OST. This work normally is done when Outlook isn't busy, but you can force Outlook to compact the OST, as Figure 1 shows. To do so in Outlook 2007, go to Tools, Account Settings. On the Data Files tab, select the OST from the list, then select Settings. On the Advanced tab, click Offline Folder File Settings, then click Compact Now. This operation can take from a few seconds to a few minutes to complete depending on the size of the OST.
Even if an OST is as small as it can be, it still might not be very efficient because internal structures within the OST degrade over time. You can run the OST Integrity Check tool (Scanost.exe), which Figure 2 shows, to check and fix any internal problems. The process is roughly equivalent to defragmenting a hard disk or running the Isinteg utility to perform integrity checks on an Exchange database. See the Microsoft article "Scan and repair corrupted Outlook data files" for details about what Scanost does and how to run it.
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