9.1.3. 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:

# 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.

• Attempts to truncate a file (to free space) or to rename it may fail:

  ...
  # 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
  

  You will have to use rm 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.