Applications that perform asynchronous cached I/O read requests and that use a disk array that has multiple spindles may encounter a low performance issue in Windows Server 2008, in Windows Essential Business Server 2008, or in Windows Vista SP1

Consider the following scenario:

• You have a storage area network (SAN) disk array or a direct attached storage (DAS) disk array that has multiple spindles.
• The disk array is installed on a computer that has one of the following Microsoft Windows operating systems installed:
  • Windows Server 2008
  • Windows Essential Business Server 2008
  • Windows Vista Service Pack 1 (SP1)
• An application uses the disk array to perform concurrent asynchronous I/O requests on this computer.

In this scenario, the application may have very low I/O read performance.

For example, you run SQL Server Analysis Service (SSAS) for an OLAP query on a Windows Server 2008-based computer. This computer has a disk array that supports concurrent I/O reads. SSAS uses 20 spindles to perform an asynchronous I/O read. In this example, you may find that the OLAP query performance in the Windows Server 2008 system is very low and that the system seems to have only 2 spindles working on the I/O request.

This problem occurs because of the asynchronous I/O mechanism in Windows Server 2008. When an I/O request is made, the asynchronous I/O mechanism in Windows Server 2008 performs the following steps:

• The cache manager finds that one of the pages that are required for the transfer is not resident in memory.
• The cache manager notifies NTFS that the page is not in memory.
• NTFS queues the page read and write request to its internal work queue and then returns STATUS_PENDING to the calling thread.
• NTFS only uses both worker threads to process its internal work queue.

Because only the two worker threads are used, you encounter the performance issue.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.

Important Windows Vista and Windows Server 2008 hotfixes are included in the same packages. However, only one of these products may be listed on the �Hotfix Request� page. To request the hotfix package that applies to both Windows Vista and Windows Server 2008, just select the product that is listed on the page.

Prerequisites

To apply this hotfix on Windows Vista-based computer, you must have Windows Vista SP1 installed. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

No prerequisites are required for Windows Server 2008-based computers.

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfixes.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

Windows Vista and Windows Server 2008 file information notes

The MANIFEST files (.manifest) and MUM files (.mum) that are installed for each environment are listed separately. MUM and MANIFEST files, and the associated security catalog (.cat) files, are critical to maintaining the state of the updated component. The security catalog files (attributes not listed) are signed with a Microsoft digital signature.

For all supported x86-based versions of Windows Server 2008 and Windows Vista

For all supported x64-based versions of Windows Server 2008 and Windows Vista

For all supported IA-64-based versions of Windows Server 2008

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For help with system performance issues in Windows Vista, visit the following Microsoft web page:

Additional file information for Windows Server 2008 and Windows Vista

Additional files for all supported x86-based versions of Windows Server 2008 and Windows Vista

Additional files for all supported x64-based versions of Windows Server 2008 and Windows Vista

Additional files for all supported IA-64-based versions of Windows Server 2008