What is Enhanced Write Filter (EWF)?

Enhanced Write Filter (EWF) is a component in XP Embedded which prevents write operations to another medium such as a DOM or Compact Flash card. EWF allows the writes to be discarded or committed to the medium later, as this minimizes writes to the specified medium. This is particularly useful if you want to prevent changes to a disk, or if the media has limited life of write-cycles which you want to manage.

How is Enhanced Write Filter functioning?

EWF provides a means for protecting a volume from writes. This allows the operating system (OS) to boot from read-only media such as CD-ROMs, write-protected hard disks, DOM or CF card. All writes to a EWF-protected volume are redirected to an overlay. These writes are cached in the overlay and made available as part of the volume. This gives the appearance that the volume is writable. The overlay may exist either on disk or in random access memory (RAM). If desired, the data stored in the overlay may be committed to the protected volume. It should be noted that if the system is shut down improperly (power failure) then all data in the overlay will be lost. Figure 1 is an overview of EWF.

The DyaloX uses RAM based EWF functionality so all data is written to RAM rather than to the disk. However, a small amount of disk space is still used to store configuration. This is a special partition on the physical disk, which is automatically created, but there needs to be unallocated space on the disk for this.

Figure 1