Microsoft Outlook 2007 uses an internal storage engine to process both personal folders (.pst files) and cached Exchange folders (.ost files). One of the biggest changes in Outlook 2007 SP2 is that this storage engine has been redesigned and improved.
When you install SP2, Outlook will rebuild .pst and .ost files so that the data adheres to a new file structure. The anatomy of this new file structure causes both .pst and .ost files to balloon to about 120% of their previous size. This means that a 2 GB .ost file would increase to about 2.4 GB in size after the service pack is applied. More than likely, that extra 400 MB probably won't complexly tax a system's disk space.
Outlook 2007 SP2's file structure
You're probably wondering why new file structure boosts performance, but also increases the size of .pst and .ost files. Disk I/O has been optimized so that data is written closer together. This improves response times since disk heads don't have to move as much during read/write operations. Microsoft also designed the storage engine to interact more efficiently with the operating system and disk-write caches.
Important: You may want to warn users that after installation, SP2 will negatively affect performance. This may sound counterproductive, especially when the service pack is designed to improve performance.
This performance dip happens because Outlook has to rebuild all of the user's .pst and .ost files. Once this process -- which can take several hours -- completes, Outlook performance will improve dramatically.
Disabling the new file structure
If you're concerned with the amount of disk space the new file structure consumes, you can configure Outlook to continue using the previous file structure. If you do this, it negates the performance gains delivered by the new file structure.
To disable the storage engine's new file structure, you must edit the registry. Be sure to perform a full system backup before making any changes to the registry.
If you want to use the old structure for .pst and .ost files, open the Registry Editor and navigate to: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST.
Create a new DWORD value called PreferCompactness, if it doesn't already exist. Set the value to 1 to use the old file structure for .pst and .ost files. If you change your mind later and want to use the new file structure, simply change the value of this key to 0 , which is the default value.
About the author: Brien M. Posey, MCSE, is a five-time recipient of Microsoft's Most Valuable Professional (MVP) award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), and File Systems and Storage. Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.
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