Moxa Technologies EM-1240-LX user manual Journaling Flash File System JFFS2

EM-1240-LX User’s Manual Software Package

The partition sizes are hard coded into the kernel binary. You must rebuild the kernel to change the partition sizes. The flash memory map is shown in the following table.

Flash Context	Flash Address		Size	Access control
Boot loader	0 – 0x3ffff	256 K		Read ONLY
Kernet &	0x40000– 0x3fffff	4	M	Read ONLY
Root File System				JFFS2
User Directory	0x400000 – 0x7fffff	4	M – 256 K	Read / Write
				JFFS2

Developers write their own programs only on partition /etc, /home, /tmp, and /usr/bin .It is advised the executed file be put in /usr/bin as this will allow developers to use hotkeys.

In addition to the flash file systems, a RAM based file system is mounted on /var/.

Journaling Flash File System (JFFS2)

The flash User Directory is formatted by the Journaling Flash File System (JFFS2), which places a compressed file system on the flash, transparent to the user.

Axis Communications in Sweden developed the Journaling Flash File System (JFFS2).

JFFS2 provides a file system directly on flash, rather than emulating a block device designed for use on flash-ROM chips. It recognizes flash-ROM chips’ special write requirements, does wear-leveling to extend flash life, keeps the flash directory structure in RAM at all times, and implements a log-structured file system that is always consistent—even if the system crashes or unexpectedly powers down. It does not require fsck on boot up.

JFFS2, the next version of JFFS, provides improved wear-leveling and garbage-collection performance, improved RAM footprint and response to system-memory pressure, improved concurrency and support for suspending flash erases, marking of bad sectors with continued use of the remaining good sectors (thus enhancing the write-life of the devices), native data compression inside the file system design; and support for hard links.

Key features of JFFS2 are:

yDirectly targeted to Flash ROM

yRobust

yConsistent across power failure

yNo integrity scan (fsck) is required at boot time after normal or abnormal shutdown

yExplicit wear leveling

yTransparent compression

Although JFFS2 is a journaling file system, this does not preclude the loss of data. The file system will remain in a consistent state across power failures, and will always be mountable. However, if the board is powered down during a write, then the incomplete write will be rolled back on the next boot. Any writes that were already completed will not be affected.

Additional information about JFFS2 is available at

http://sources.redhat.com/jffs2/jffs2.pdf

http://developer.axis.com/software/jffs/

http://www.linux-mtd.infradead.org/

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