EWF API (Standard 7 SP1)


The Enhanced Write Filter (EWF) API exposes a set of functions that enable an application to interact with EWF.

To enable support for the Enhanced Write Filter API, add the Enhanced Write Filter API component to your run-time image.

EWF is used to protect one or more disk volumes by intercepting write requests to the volume and redirecting them to an overlay volume.


Windows Embedded Standard 7 only supports EWF RAM and RAM Reg modes. Disk mode is not supported. The API remains unchanged, but functions that attempt to use disk mode are disabled.

The EWF API functions enable applications to query and modify EWF configuration settings for protected volumes. The following list shows the typical usage scenarios for these functions:

  • Disabling/enabling EWF for a protected volume
  • Restoring or discarding an overlay level
  • Creating a checkpoint by adding an overlay level
  • Committing the current overlay
  • Persisting data through a restart
  • Enabling Hibernate Once/Resume Many (HORM)

EWF Configuration Settings

Typically, configuration settings for EWF are stored in the EWF volume. For RAM Reg EWF mode, an EWF volume store may not exist as a physical disk volume. For these configurations, all overlay configuration data is stored and retrieved from the registry.The following list shows the two primary configuration structures that the EWF API exposes:

EWF API Considerations

The following list shows information that is important to consider before you use the API:

  • Many of the EWF functions do not take effect until the system restarts. Review the function documentation to make sure that you understand the expected behavior.
  • EWF offers functions to get and set persistent data. These functions are useful to persist data through a restart.
  • Devices without a volume store can only disable EWF by committing the current RAM overlay to the protected volume at the same time.

See Also


Configure EWF RAM Mode
Configure EWF RAM Reg Mode


EWF API Functions
EWF API Structures
EWF API Code Sample