FlashFilesystemsJFFS2 < DULG

%SECTION0{name=FlashFilesystemsJffs2}% Second Version of JFFS

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:

<verbatim>
# 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
</verbatim>

%X% 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.

   * Attempts to truncate a file (to free space) or to rename it may
     fail:
     <verbatim>
...
# cp /bin/bash file
cp: writing `file': No space left on device
# >file
bash: file: No space left on device
# mv file foo
mv: cannot create regular file `foo': No space left on device
     </verbatim>
     You will have to use <command>rm</command> to actually delete a
     file in this situation.

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.
9.1.2. Journalling Flash File System		1. Abstract		9.1.4. Compressed ROM Filesystem
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