QNX provides a number of options for persistent data storage, with support for flash devices, disks, network file systems, CD-ROM, DVDs and RAM disks. All filesystems are accessible through a simple POSIX / C API (open, read, write, close, ...) for true portability. In the QNX® Neutrino® RTOS, file systems execute outside the kernel, in memory-protected user space. As a result, developers can start, stop, or upgrade file systems on the fly, without having to reboot.

Moreover, multiple file systems can run concurrently on the same target, and even work in concert to extend one another's capabilities. For example, the compression file system can work with a flash file system dramatically reduce the flash memory requirements on a given target.

File systems at a glance

Embedded file systems — NOR and NAND Flash

QNX flash file system technology delivers reliability at the file system level, through read/write persistent storage that resists data corruption from unexpected power loss. The QNX implementation also provides wear-level capability, to increase the life span of flash parts. NOR flash devices are supported with QNX Flash File System version 3 (FFSv3). NAND flash devices are supported with the Embedded Transaction File System (ETFS) technology.

Technology overview

NOR and NAND devices benefit from the following technologies:

• Resist power failure corruption
  
  • System automatically detects and corrects file system corruption caused by unexpected rest/power loss
    
• POSIX, ISO C file semantics and tools
  
  • Dynamic creation and manipulation of files and directories
    
  • Symbolic names, permissions and long file names
    
  • Standard file stream operations
    
  • Raw flash can be read/written directly for reprogramming boot loaders via standard POSIX command line tools such as cp and dd
    
• File system performance
  
  • Background reclaim concurrently performs background garbage collection to reclaim unused space, mitigating potential I/delays to find and erase blocks and sectors
    
  • Files and directories are accessible in a much shorter time, allowing faster application loading.
    
  • RAM file system for caching and temporary files
    
• Wear leveling
  
  • Extends flash lifespan by distributing usage across the entire flash part
    
• Compression support to optimize use of flash
  

NOR flash devices (QNX Flash File System version 3)

• Fault recovery - Updates to metadata are handled in carefully executed sequences. This allows for high file system integrity even after power-loss. After a reset or power-on, the file system is scanned and data integrity is restored where possible.
  
• Bad sector handling - Sectors with errors are recorded and transparently avoided.
  

NAND flash devices (QNX Embedded Transaction File System)

• Resist power failure corruptions - Atomic transactions ensure integrity of file and directory operations tresist power failure corruptions
  
• Bad block support - Factory-marked bad blocks are reserved and unused.
  
• Read-degradation monitoring - Blocks are marked for refresh before hitting the normal read degradation limit. This avoids possible ECC errors and the need to recover data.
  
• Automatic file de-fragmentation - Write buffering is used tconsolidate small write transactions before they are written tNAND; this helps avoid fragmentation. Background de-fragmentation happens automatically and is automatically preempted by user activity.
  
• Bulk copy support - File system data is not block specific, therefore NAND devices can be bulk copied using a separate flash programmer.