DA-660-8/16-LX User’s Manual
Introduction
1-3
In order to prevent user applications from crashing the
Root File System, the DA-660 uses a specially designed
Mini File System with Protected Configuration
for
emergency use. This
Mini File System
comes with serial
and Ethernet communication capability for users to load
the
Factory Default Image
file. The Mini File System
will only be activated if the boot loader fails to load the
User Root File System.
User AP
User Directory
(User Configuration)
Mini Root File System
Configuration
Linux Kernel & Root
HW
Boot Loader
To improve system reliability, the DA-660 has a built-in mechanism that prevents the system from
crashing. The procedure is as follows.
When the Linux kernel boots up, the kernel will mount the root file system, and then enable
services and daemons. During this time, the kernel will start searching for system configuration
parameters via
rc
or
inittab
.
Normally, the kernel uses the User Root File System to boot up the system. The Mini Root File
System is protected, and cannot be changed by the user, and thus provides a safe zone. The kernel
will only use the Mini Root File System when the User Root File System crashes.
For more information about the memory map and programming in DA-660, refer to Chapter 5,
Programmer’s Guide.
Journaling Flash File System (JFFS2)
The User Root File System in the flash memory is formatted with the
Journaling Flash File
System (JFFS2)
. The formatting process places a compressed file system in the flash memory,
transparent to the user.
The Journaling Flash File System (JFFS2), which was developed by Axis Communications in
Sweden, places 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.
The system is always consistent, even if it encounters crashes or improper power-downs, and 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 that enhances 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:
y
Targets the Flash ROM directly
y
Robustness
y
Consistency across power failures
y
No integrity scan (fsck) is required at boot time after normal or abnormal shutdown
y
Explicit wear leveling
y
Transparent compression
Although JFFS2 is a journaling file system, it does not exclude 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 operation, then the incomplete write operation will be
rolled back on the next boot, but write operations that have already been completed will not be
