Increased Performance Using FILE_FLAG_SEQUENTIAL_SCAN

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1, 3.5, and 3.51
      - Microsoft Windows 95 version 4.0
  

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 "mostly" 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 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.