Microsoft KB Archive/98756

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Increased Performance Using FILE_FLAG_SEQUENTIAL_SCAN

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Q98756

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The information in this article applies to:


 * Microsoft Win32 Application Programming Interface (API), included with:
 * the operating system: Microsoft Windows NT, versions 3.1, 3.5, 3.51
 * Microsoft Windows 95

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SUMMARY
There is a flag for CreateFile called FILE_FLAG_SEQUENTIAL_SCAN which will direct the Cache Manager to access the file sequentially.

Anyone reading potentially large files with sequential access can specify this flag for increased performance. This flag is useful if you are reading files that are &quot;mostly&quot; sequential, but you occasionally skip over small ranges of bytes.

MORE INFORMATION
The effect on the Cache Manager of this flag is two-fold:


 * There is a minor savings because the Cache Manager dispenses with keeping a history of reads on the file, and tries to maintain a high-water mark on read ahead, which is always a certain delta from the most recent read.
 * More importantly, the Cache Manager reads further ahead for sequential access files--currently about three times more than files that are currently detected for sequential access.

If the caller makes multiple passes through a file, there are no negative effects of specifying the sequential flag, because the Cache Manager will still disable read ahead for as long as the application is getting hits on the file (such as on the second or subsequent pass).

If you are working on an application where your ability to sequentially read file data is key to performance, you may want to consider adding the sequential flag to your create file call. This is especially true of applications that use this flag to read from a CD-ROM.

Additional query words: 3.10 3.50 4.00

Keywords : kbOSWinNT310 kbOSWinNT350 kbOSWinNT351 kbOSWin95

Issue type :

Technology : kbAudDeveloper kbWin32sSearch kbWin32API