Probably even more interesting for embedded systems is the second
version of
JFFS,
JFFS2, since it not only fixes a few design issues
with
JFFS, but also adds transparent compression, so that you can save
a lot of precious flash memory.
The
mkfs.jffs2
tool is used to create a JFFS2 filesystem image; it
populates the image with files from a given directory. For instance,
to create a JFFS2 image for a flash partition of 3 MB total size and
to populate it with the files from the
/tmp/flashtools directory you
would use:
# mkfs.jffs2 --pad=3145728 --eraseblock=262144 \
--root=/tmp/flashtools/ --output image.jffs2
# eraseall /dev/mtd4
Erased 3072 Kibyte @ 0 -- 100% complete.
\# dd if=image.jffs2 of=/dev/mtd4 bs=256k
12+0 records in
12+0 records out
# mount -t jffs2 /dev/mtdblock4 /mnt
# df /mnt
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/mtdblock4 3072 2488 584 81% /mnt

Note: Especially when you are running time-critical applications
on your system you should carefully study if the behaviour of the
flash filesystem might have any negative impact on your application.
After all, a flash device is not a normal harddisk. This is especially
important when your flash filesystem gets full; JFFS2 acts a bit weird
then:
- You will note that an increasing amount of CPU time is spent by
the filesystem's garbage collection kernel thread.
- Access times to the files on the flash filesystem may increase
drastically.
This is especially critical when you are using the flash filesystem to
store log files: when your application detects some abnormal condition
and produces lots of log messages (which usually are especially
important in this situation) the filesystem may fill up and cause
extreme long delays - if your system crashes, the most important
messages may never be logged at all.