FBWF Features (Standard 8)
7/8/2014
Review the features available in the File-Based Write Filter (FBWF) for Windows Embedded 8 Standard (Standard 8).
File-Based Write Filter (FBWF) includes write filter transparency to applications; selective write-through, commit, and restore; improved overlay memory use; and enhanced APIs.
File and Directory Management Transparency
Applications and OS components make file and directory management calls without being affected by the write caching provided by File-Based Write Filter (FBWF). FBWF provides a composite view of the file system without imposing limitations on user mode applications. Applications perform explicit file I/O through functions such as CreateDirectory, CreateFile, ReadFile, and WriteFile. These calls appear to interact with the file system but may result in overlay cache I/O.
The following directory management operations may result in error:
- Moving files across protection boundaries, for example, from a protected volume to an unprotected volume.
- Committing deleted files or new directories.
FBWF supports NTFS, FAT32, and exFAT file systems.
FBWF can protect formatted volumes only. FBWF cannot recognize unformatted volumes.
Files in FBWF overlay cache are not encrypted.
Selective Write Through
You can select files and directories to be excluded from the overlay and written directly to the protected volume. Files and directories that are "write-through" are selected by using the following:
- Settings for the FBWF module in Image Configuration Editor (ICE) during the OS build process.
- File-Based Write Filter (FBWF) APIs at run time.
- FBWF Manager at run time in the development environment.
Selective Commits and Restores
You can select files (or file changes) to be moved from the overlay to the protected volume. Files or changes moved to the protected volume take effect immediately and persist through restarts. You can also discard changed files from the overlay and restore the view of those files from the underlying volume.
Both functions are useful for devices that are rarely restarted or devices that are shared across users.
The commit functionality has the following limitations:
- You can only commit one file at a time to an existing folder.
- You cannot commit deleted files.
- You cannot commit folders.
FBWF Memory Optimization
File-Based Write Filter (FBWF) conserves and reclaims memory in overlay cache. For example, FBWF frees overlay memory when files are deleted or reduced in size.
You can configure FBWF to use one of the following cache types:
- Pre-allocated - In this mode, FBWF sets aside memory during initialization equal to the specified overlay size. FBWF frees memory within its pre-allocated cache, so that only FBWF can use the reclaimed memory.
- Dynamic - In this mode, FBWF allocates memory when it is needed up to a limit equal to the overlay size. FBWF frees memory that can be used by all processes and services on the system.
- Dynamic-Compressed - Same as Dynamic but the overlay is compressed to save space.