Moxa W321-LX [7/80] Journaling flash file system jffs2

Moxa W341-LX [7/80] Journaling flash file system jffs2
W321/341 Linux Introduction
1-3
Because the root file system is protected and cannot be changed by the user, a “safe” zone is created which
makes it very difficult for the base OS to become corrupted by anything other than hardware faults. Generally
speaking, user behavior or normal read-write operations will not affect the root OS.
For more information about the memory map and programming, refer to Chapter 6 in the Programmer’s
Guide.
Journaling Flash File System (JFFS2)
The root file system and user directories in the flash memory are formatted with the Journaling Flash File
System (JFFS2). This file system creates a compressed file tree in the flash memory that is is completely
transparent to the user, and behaves as if it were an ordinary file sytem installed on a hard disk.
The Journaling Flash File System (JFFS2) was developed by Axis Communications in Sweden; it creates a file
system directly on the flash, instead of emulating a block device. It is designed for use on flash-ROM chips and
recognizes the special write requirements of a flash-ROM chip. JFFS2 implements wear-leveling to extend the
life of the flash disk, and stores the flash directory structure in the RAM. A log-structured file system is
maintained at all times, so that the system is always consistent, even if it crashes or other improper
power-downs. Because of these featuers, JFFS2 does not require fsck (file system check) on boot-up.
JFFS2 is the newest version of JFFS. It 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 (enhancing
the write-life of the devices), native data compression inside the file system design, and support for hard links.
The key features of JFFS2 are:
It targets the Flash ROM directly
Robustness
Consistency across power failures
No integrity scan (fsck) is required at boot time, or after either normal or abnormal shutdown
Explicit wear leveling
Transparent 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; only completed writes 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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