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.pst and .ost files may be up to 20% larger after the Outlook 2007 hotfix package dated February 24, 2009

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Important This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows XP and Windows Vista
After you install the February 24, 2009 cumulative update for Outlook 2007 you may see the file size of your Outlook offline data (.ost) and Outlook data (.pst) files grow by up to 20%. This same behavior is also found in later versions of Microsoft Outlook.
The February 24, 2009 cumulative update for Outlook 2007 includes changes related to the general reading and writing characteristics of the Outlook data (.pst) and Outlook offline data (.ost) files. This usually results in lowered I/O demand from your hard disk to perform the same general operations in Outlook. For example, the file flushing process is optimized so that it interacts more efficiently with the operating system and disk-write caches. There is also a change in the way new data is written to the .pst and .ost files so that the data is saved much closer together on the physical disk. Therefore, there is less physical work that is required from the disk. This reduces the time that is required to write the same amount of data to the disk drive. However, a side effect of this data writing algorithm is that .pst and .ost files are about 20% larger than they were before the February 24, 2009 cumulative update. 
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

If you do not want your .pst and .ost files to use the new process, disable the feature by adding the following registry data to the Outlook client:

Key: HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\PST 
where xx.0 is 12.0 for Outlook 2007 and 14.0 for Outlook 2010.
DWORD: PreferCompactness
Values: 0 = (the default) new behavior
            1 = disable the new behavior

Note When you use Cached Exchange Mode to connect to your Microsoft Exchange Server mailbox, mailbox data is cached locally in the Outlook offline data (.ost) file. In many cases, the size of the .ost file is larger than the size of the Exchange Server mailbox that is being cached. However, if it seems that the size of the .ost file is growing at a faster rate than the Exchange mailbox, you can troubleshoot the issue by setting the PreferCompactness registry value to 1. Outlook may be unable to compact the existing .ost file fast enough to prevent the size from increasing. Therefore, replace the existing .ost file or create a new Outlook profile to force Outlook to create a new .ost file.
More Information
For additional information on the February 24, 2009 Cumulative Update for Outlook 2007, please see the following articles in the Microsoft Knowledge Base:
961752 Description of the Outlook 2007 hotfix package (Outlook.msp): February 24, 2009 
968009 Outlook 2007 improvements in the February 2009 cumulative update 
For more information about excluding .pst and .ost data files from file level antivirus scanning, visit the following Microsoft website:


Article ID: 968413 - Last Review: 01/16/2015 11:03:50 - Revision: 4.0

Microsoft Office Outlook 2007, Microsoft Outlook 2010

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