JFFS2/Mounting

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[edit] Purpose

There are a few reasons why this might be useful. Transparent Runtime Compression on a block device is one. Working with embedded systems is another.

[edit] Prepare the system

[edit] Emerge proper tools

This package didn't compile at the time of writing, because it was over 2 years old. 

emerge sys-fs/mtd

An alternative is to download a binary of mkfs.jffs2 v1.17 for i386 Linux here. Another alternative is checkout the sources from the cvs at [1] Another alternative is to use sys-fs/mtd-utils since it will replace sys-fs/mtd, http://bugs.gentoo.org/show_bug.cgi?id=203279. 

ACCEPT_KEYWORDS="~x86" emerge sys-fs/mtd-utils

[edit] Build your kernel with the proper MTD devices enabled

These modules need to be loaded (you can put them in modules.autoload) or just compile them right in.

  • "Device Drivers"
    • "Block Devices"
      • "Loopback Device Support" (loop)
    • "Memory Technology Device (MTD) support"
      • "Caching block device access to MTD devices" (mtdblock module) (required for block2mtd)
      • "Self-contained MTD device drivers..."
        • "MTD emulation using block device" (block2mtd),
  • "File Systems"
    • "Miscellaneous Filesystems"
      • "Journalling Flash File System v2 (JFFS2) support"
        • "Advanced compression options for JFFS2"
          • "JFFS2 ZLIB compression support"

Reboot the system now and ensure the modules are loaded

[edit] Make the device node block device

mknod /dev/mtdblock0 b 31 0

[edit] Make the initial image

This creates an empty folder that will become the root of the filesystem 

mkdir /root/empty

Makes a jffs2 image, the --eraseblock=256000000 is number of bytes, in later versions of mkfs.jffs2 you can use sci notation 

mkfs.jffs2 --root=/root/empty --eraseblock=256000000 --pad -o /jffs2drive.bin

[edit] Mount the image

Uses a loop to make the jffs2 image file a block device. 

losetup /dev/loop0 /jffs2drive.bin or
losetup -o 16 /dev/loop0 /jffs2drive.bin

Tell the block2mtd driver what you want it to convert to a fake MTD device. 

echo "/dev/loop0" > /sys/module/block2mtd/parameters/block2mtd

This will show if you have things set up right (size should be non-zero) 

cat /proc/mtd

Finally mount the filesystem with the jffs2 filesystem type 

mount -t jffs2 /dev/mtdblock0 /mnt/jffs2/

[edit] Block device disk speed comparisons

These were performed multiple times, the times represent the actual time of a typical run. Resier3 mounted loopback 

centauri reiser3 # time tar xjf /usr/portage/distfiles/linux-2.6.17.tar.bz2
real    0m35.826s
user    0m27.710s
sys     0m5.252s

JFFS2 mounted loopback AND block2mtd. The additional time in sys is probably mostly due to the runtime compression. 

centauri jffs2 # time tar xjf /usr/portage/distfiles/linux-2.6.17.tar.bz2
real    1m0.334s
user    0m28.594s
sys     0m24.894s
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