Issues with RAM-based Enhanced Write Filter (EWF) overlays

Summary

This article describes how to avoid memory-related problems that may occur on runtime XP embedded images that are built with the RAM-based Enhanced Write Filter (EWF) feature.

For example, during a problem like this, you may receive the following error message:
Windows - Delayed Write Failed. Windows was unable to save all data for the file .. The data has been lost. This error may be caused by a failure of your computer hardware or network connection...

More Information

The Enhanced Write Filter (EWF) is a component in Windows XP Embedded that protects the contents of a volume on the destination media by redirecting all writes to another storage location, which is known as an overlay.

For each protected volume, you can retain the overlay information in either of the following ways:
  • In a disk overlay on a predesignated disk partition that is known as the EWF volume. With a disk overlay, the information is retained after you restart the computer.
  • In RAM overlay, which uses system memory. All overlay information is lost when you shut down or restart the target computer.
When you use a RAM-based EWF overlay, the EWF driver must have enough memory to work properly. RAM-based EWF is not appropriate for every configuration, especially those that generate many disk write requests to the protected volume. For each write request to the protected volume, there is an associated consumption of memory because disk changes are stored in memory. This memory is allocated from the system non-paged pool, which is limited to 256 megabytes (MB). If the operating system has enough physical memory, this limitation is resolved by installing Service Pack 1.

The following methods can help avoid memory-related problems on RAM-based EWF:

  • In the Enhanced Write Filter component in your configuration, on the EWF Configuration Setting tab, click the Use Less Overlay Space and less Writes check box.
  • Rule out any unnecessary disk I/O (especially write requests).
  • Make sure that the disk defragmenting tool is not running in the background.
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ID articol: 328620 - Ultima examinare: 29 oct. 2008 - Revizie: 1

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