[net-next-2.6.git] / Documentation / filesystems / ubifs.txt

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
Commit	Line	Data
e56a99d5 AB	1	Introduction
	2	=============
	3
	4	UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
	5	Block Images". UBIFS is a flash file system, which means it is designed
	6	to work with flash devices. It is important to understand, that UBIFS
	7	is completely different to any traditional file-system in Linux, like
	8	Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
	9	which work with MTD devices, not block devices. The other Linux
	10	file-system of this class is JFFS2.
	11
	12	To make it more clear, here is a small comparison of MTD devices and
	13	block devices.
	14
	15	1 MTD devices represent flash devices and they consist of eraseblocks of
	16	rather large size, typically about 128KiB. Block devices consist of
	17	small blocks, typically 512 bytes.
	18	2 MTD devices support 3 main operations - read from some offset within an
	19	eraseblock, write to some offset within an eraseblock, and erase a whole
	20	eraseblock. Block devices support 2 main operations - read a whole
	21	block and write a whole block.
	22	3 The whole eraseblock has to be erased before it becomes possible to
	23	re-write its contents. Blocks may be just re-written.
	24	4 Eraseblocks become worn out after some number of erase cycles -
	25	typically 100K-1G for SLC NAND and NOR flashes, and 1K-10K for MLC
	26	NAND flashes. Blocks do not have the wear-out property.
	27	5 Eraseblocks may become bad (only on NAND flashes) and software should
	28	deal with this. Blocks on hard drives typically do not become bad,
	29	because hardware has mechanisms to substitute bad blocks, at least in
	30	modern LBA disks.
	31
	32	It should be quite obvious why UBIFS is very different to traditional
	33	file-systems.
	34
	35	UBIFS works on top of UBI. UBI is a separate software layer which may be
	36	found in drivers/mtd/ubi. UBI is basically a volume management and
	37	wear-leveling layer. It provides so called UBI volumes which is a higher
	38	level abstraction than a MTD device. The programming model of UBI devices
	39	is very similar to MTD devices - they still consist of large eraseblocks,
	40	they have read/write/erase operations, but UBI devices are devoid of
	41	limitations like wear and bad blocks (items 4 and 5 in the above list).
	42
	43	In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
	44	very different and incompatible to JFFS2. The following are the main
	45	differences.
	46
	47	* JFFS2 works on top of MTD devices, UBIFS depends on UBI and works on
	48	top of UBI volumes.
	49	* JFFS2 does not have on-media index and has to build it while mounting,
	50	which requires full media scan. UBIFS maintains the FS indexing
	51	information on the flash media and does not require full media scan,
	52	so it mounts many times faster than JFFS2.
	53	* JFFS2 is a write-through file-system, while UBIFS supports write-back,
	54	which makes UBIFS much faster on writes.
	55
	56	Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
	57	it possible to fit quite a lot of data to the flash.
	58
	59	Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.
2e244d08	60	It does not need stuff like fsck.ext2. UBIFS automatically replays its
e56a99d5 AB	61	journal and recovers from crashes, ensuring that the on-flash data
	62	structures are consistent.
	63
	64	UBIFS scales logarithmically (most of the data structures it uses are
	65	trees), so the mount time and memory consumption do not linearly depend
	66	on the flash size, like in case of JFFS2. This is because UBIFS
	67	maintains the FS index on the flash media. However, UBIFS depends on
	68	UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
	69	Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
	70
	71	The authors of UBIFS believe, that it is possible to develop UBI2 which
	72	would scale logarithmically as well. UBI2 would support the same API as UBI,
	73	but it would be binary incompatible to UBI. So UBIFS would not need to be
	74	changed to use UBI2
	75
	76
	77	Mount options
	78	=============
	79
	80	(*) == default.
	81
4793e7c5 AH	82	bulk_read read more in one go to take advantage of flash
	83	media that read faster sequentially
	84	no_bulk_read (*) do not bulk-read
2953e73f AH	85	no_chk_data_crc skip checking of CRCs on data nodes in order to
	86	improve read performance. Use this option only
	87	if the flash media is highly reliable. The effect
	88	of this option is that corruption of the contents
	89	of a file can go unnoticed.
	90	chk_data_crc (*) do not skip checking CRCs on data nodes
80736d41 AB	91	compr=none override default compressor and set it to "none"
	92	compr=lzo override default compressor and set it to "lzo"
	93	compr=zlib override default compressor and set it to "zlib"
e56a99d5 AB	94
	95
	96	Quick usage instructions
	97	========================
	98
	99	The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
	100	where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
	101	UBI volume name.
	102
	103	Mount volume 0 on UBI device 0 to /mnt/ubifs:
	104	$ mount -t ubifs ubi0_0 /mnt/ubifs
	105
	106	Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
	107	name):
	108	$ mount -t ubifs ubi0:rootfs /mnt/ubifs
	109
	110	The following is an example of the kernel boot arguments to attach mtd0
	111	to UBI and mount volume "rootfs":
	112	ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
	113
	114
	115	Module Parameters for Debugging
	116	===============================
	117
	118	When UBIFS has been compiled with debugging enabled, there are 3 module
	119	parameters that are available to control aspects of testing and debugging.
	120	The parameters are unsigned integers where each bit controls an option.
	121	The parameters are:
	122
	123	debug_msgs Selects which debug messages to display, as follows:
	124
	125	Message Type Flag value
	126
	127	General messages 1
	128	Journal messages 2
	129	Mount messages 4
	130	Commit messages 8
	131	LEB search messages 16
	132	Budgeting messages 32
	133	Garbage collection messages 64
	134	Tree Node Cache (TNC) messages 128
	135	LEB properties (lprops) messages 256
	136	Input/output messages 512
	137	Log messages 1024
	138	Scan messages 2048
	139	Recovery messages 4096
	140
	141	debug_chks Selects extra checks that UBIFS can do while running:
	142
	143	Check Flag value
	144
	145	General checks 1
	146	Check Tree Node Cache (TNC) 2
	147	Check indexing tree size 4
	148	Check orphan area 8
	149	Check old indexing tree 16
	150	Check LEB properties (lprops) 32
	151	Check leaf nodes and inodes 64
	152
	153	debug_tsts Selects a mode of testing, as follows:
	154
	155	Test mode Flag value
	156
	157	Force in-the-gaps method 2
158	Failure mode for recovery testing 4
159
160	For example, set debug_msgs to 5 to display General messages and Mount
161	messages.
162
163
164	References
165	==========
166
167	UBIFS documentation and FAQ/HOWTO at the MTD web site:
168	http://www.linux-mtd.infradead.org/doc/ubifs.html
169	http://www.linux-mtd.infradead.org/faq/ubifs.html