Introduction
  =============
  
  UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
  Block Images". UBIFS is a flash file system, which means it is designed
  to work with flash devices. It is important to understand, that UBIFS
  is completely different to any traditional file-system in Linux, like
  Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
  which work with MTD devices, not block devices. The other Linux
  file-system of this class is JFFS2.
  
  To make it more clear, here is a small comparison of MTD devices and
  block devices.
  
  1 MTD devices represent flash devices and they consist of eraseblocks of
    rather large size, typically about 128KiB. Block devices consist of
    small blocks, typically 512 bytes.
  2 MTD devices support 3 main operations - read from some offset within an
    eraseblock, write to some offset within an eraseblock, and erase a whole
    eraseblock. Block  devices support 2 main operations - read a whole
    block and write a whole block.
  3 The whole eraseblock has to be erased before it becomes possible to
    re-write its contents. Blocks may be just re-written.
  4 Eraseblocks become worn out after some number of erase cycles -
    typically 100K-1G for SLC NAND and NOR flashes, and 1K-10K for MLC
    NAND flashes. Blocks do not have the wear-out property.
  5 Eraseblocks may become bad (only on NAND flashes) and software should
    deal with this. Blocks on hard drives typically do not become bad,
    because hardware has mechanisms to substitute bad blocks, at least in
    modern LBA disks.
  
  It should be quite obvious why UBIFS is very different to traditional
  file-systems.
  
  UBIFS works on top of UBI. UBI is a separate software layer which may be
  found in drivers/mtd/ubi. UBI is basically a volume management and
  wear-leveling layer. It provides so called UBI volumes which is a higher
  level abstraction than a MTD device. The programming model of UBI devices
  is very similar to MTD devices - they still consist of large eraseblocks,
  they have read/write/erase operations, but UBI devices are devoid of
  limitations like wear and bad blocks (items 4 and 5 in the above list).
  
  In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
  very different and incompatible to JFFS2. The following are the main
  differences.
  
  * JFFS2 works on top of MTD devices, UBIFS depends on UBI and works on
    top of UBI volumes.
  * JFFS2 does not have on-media index and has to build it while mounting,
    which requires full media scan. UBIFS maintains the FS indexing
    information on the flash media and does not require full media scan,
    so it mounts many times faster than JFFS2.
  * JFFS2 is a write-through file-system, while UBIFS supports write-back,
    which makes UBIFS much faster on writes.
  
  Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
  it possible to fit quite a lot of data to the flash.
  
  Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.

  It does not need stuff like fsck.ext2. UBIFS automatically replays its

  journal and recovers from crashes, ensuring that the on-flash data
  structures are consistent.
  
  UBIFS scales logarithmically (most of the data structures it uses are
  trees), so the mount time and memory consumption do not linearly depend
  on the flash size, like in case of JFFS2. This is because UBIFS
  maintains the FS index on the flash media. However, UBIFS depends on
  UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
  Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
  
  The authors of UBIFS believe, that it is possible to develop UBI2 which
  would scale logarithmically as well. UBI2 would support the same API as UBI,
  but it would be binary incompatible to UBI. So UBIFS would not need to be
  changed to use UBI2
  
  
  Mount options
  =============
  
  (*) == default.

  bulk_read		read more in one go to take advantage of flash
  			media that read faster sequentially
  no_bulk_read (*)	do not bulk-read

  no_chk_data_crc		skip checking of CRCs on data nodes in order to
  			improve read performance. Use this option only
  			if the flash media is highly reliable. The effect
  			of this option is that corruption of the contents
  			of a file can go unnoticed.
  chk_data_crc (*)	do not skip checking CRCs on data nodes

  compr=none              override default compressor and set it to "none"
  compr=lzo               override default compressor and set it to "lzo"
  compr=zlib              override default compressor and set it to "zlib"

  
  
  Quick usage instructions
  ========================
  
  The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
  where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
  UBI volume name.
  
  Mount volume 0 on UBI device 0 to /mnt/ubifs:
  $ mount -t ubifs ubi0_0 /mnt/ubifs
  
  Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
  name):
  $ mount -t ubifs ubi0:rootfs /mnt/ubifs
  
  The following is an example of the kernel boot arguments to attach mtd0
  to UBI and mount volume "rootfs":
  ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
  
  
  Module Parameters for Debugging
  ===============================
  
  When UBIFS has been compiled with debugging enabled, there are 3 module
  parameters that are available to control aspects of testing and debugging.
  The parameters are unsigned integers where each bit controls an option.
  The parameters are:
  
  debug_msgs	Selects which debug messages to display, as follows:
  
  		Message Type				Flag value
  
  		General messages			1
  		Journal messages			2
  		Mount messages				4
  		Commit messages				8
  		LEB search messages			16
  		Budgeting messages			32
  		Garbage collection messages		64
  		Tree Node Cache (TNC) messages		128
  		LEB properties (lprops) messages	256
  		Input/output messages			512
  		Log messages				1024
  		Scan messages				2048
  		Recovery messages			4096
  
  debug_chks	Selects extra checks that UBIFS can do while running:
  
  		Check					Flag value
  
  		General checks				1
  		Check Tree Node Cache (TNC)		2
  		Check indexing tree size		4
  		Check orphan area			8
  		Check old indexing tree			16
  		Check LEB properties (lprops)		32
  		Check leaf nodes and inodes		64
  
  debug_tsts	Selects a mode of testing, as follows:
  
  		Test mode				Flag value
  
  		Force in-the-gaps method		2
  		Failure mode for recovery testing	4
  
  For example, set debug_msgs to 5 to display General messages and Mount
  messages.
  
  
  References
  ==========
  
  UBIFS documentation and FAQ/HOWTO at the MTD web site:
  http://www.linux-mtd.infradead.org/doc/ubifs.html
  http://www.linux-mtd.infradead.org/faq/ubifs.html