The creation of a JFFS2 image is fairly simple:
$ cd ${PRJROOT}
$ mkfs.jffs2 -r rootfs/ -o images/rootfs-jffs2.img
We use the -r option to specify the location of
the directory containing the root filesystem, and the
-o option to specify the name of the output file
where the filesystem image should be stored. In addition to these
options, we could use -l or
-b to create little endian or big endian images,
respectively. The JFFS2 compression ratio is much smaller than
CRAMFS. For a root filesystem containing 7840 KB, for example, the
resulting JFFS2 image is 6850 KB in size. The compression ratio is a
little above
10%.
Once you create the JFFS2 image, you
can write it to its designated MTD device. If this device is
accessible on the host, you can carry out the appropriate commands
directly on the host. Otherwise, follow the instructions in Section 8.2: boot your target with an
NFS-mounted root filesystem, place the JFFS2 image on that
filesystem, and issue the commands on the target to write the image
to the designated MTD device. Regardless of your setup, you first
need to erase the MTD device where the image will be placed:
# eraseall /dev/mtd5 Erased 8192 Kibyte @ 0 -- 100% complete.
Obviously, the space available on the MTD storage device must be
equal to or larger than the JFFS2 image you are placing on it. With
the MTD device erased, copy the JFFS2 image to the MTD partition:
# cat images/rootfs-jffs2.img > /dev/mtd5
Now, mount the copied filesystem to
take a look at it:
# mount -t jffs2 /dev/mtdblock5 /mnt # mount ... /dev/mtdblock5 on /mnt type jffs2 (rw) # ls mnt bin etc linuxrc sbin usr dev lib proc tmp var # umount mnt
Unlike disk filesystems, JFFS2
cannot be mounted on loopback using a mount -o loop
... command to view its content. Instead, it must be
mounted from a real MTD device as done above. If you have no real MTD
device on your host, such as CFI flash, you could use the virtual
memory MTD device presented in Chapter 3. You
could also use the jffs2reader command
introduced in the previous chapter to view the
image's content.
If your
target had previously been using an NFS-mounted root filesystem, you
are now ready to boot it using the JFFS2 filesystem as its root
filesystem.
