1Introduction
2=============
3
4UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
5Block Images". UBIFS is a flash file system, which means it is designed
6to work with flash devices. It is important to understand, that UBIFS
7is completely different to any traditional file-system in Linux, like
8Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
9which work with MTD devices, not block devices. The other Linux
10file-system of this class is JFFS2.
11
12To make it more clear, here is a small comparison of MTD devices and
13block devices.
14
151 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.
182 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.
223 The whole eraseblock has to be erased before it becomes possible to
23  re-write its contents. Blocks may be just re-written.
244 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.
275 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
32It should be quite obvious why UBIFS is very different to traditional
33file-systems.
34
35UBIFS works on top of UBI. UBI is a separate software layer which may be
36found in drivers/mtd/ubi. UBI is basically a volume management and
37wear-leveling layer. It provides so called UBI volumes which is a higher
38level abstraction than a MTD device. The programming model of UBI devices
39is very similar to MTD devices - they still consist of large eraseblocks,
40they have read/write/erase operations, but UBI devices are devoid of
41limitations like wear and bad blocks (items 4 and 5 in the above list).
42
43In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
44very different and incompatible to JFFS2. The following are the main
45differences.
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
56Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
57it possible to fit quite a lot of data to the flash.
58
59Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.
60It does not need stuff like fsck.ext2. UBIFS automatically replays its
61journal and recovers from crashes, ensuring that the on-flash data
62structures are consistent.
63
64UBIFS scales logarithmically (most of the data structures it uses are
65trees), so the mount time and memory consumption do not linearly depend
66on the flash size, like in case of JFFS2. This is because UBIFS
67maintains the FS index on the flash media. However, UBIFS depends on
68UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
69Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
70
71The authors of UBIFS believe, that it is possible to develop UBI2 which
72would scale logarithmically as well. UBI2 would support the same API as UBI,
73but it would be binary incompatible to UBI. So UBIFS would not need to be
74changed to use UBI2
75
76
77Mount options
78=============
79
80(*) == default.
81
82bulk_read		read more in one go to take advantage of flash
83			media that read faster sequentially
84no_bulk_read (*)	do not bulk-read
85no_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.
90chk_data_crc		do not skip checking CRCs on data nodes
91compr=none              override default compressor and set it to "none"
92compr=lzo               override default compressor and set it to "lzo"
93compr=zlib              override default compressor and set it to "zlib"
94
95
96Quick usage instructions
97========================
98
99The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
100where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
101UBI volume name.
102
103Mount volume 0 on UBI device 0 to /mnt/ubifs:
104$ mount -t ubifs ubi0_0 /mnt/ubifs
105
106Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
107name):
108$ mount -t ubifs ubi0:rootfs /mnt/ubifs
109
110The following is an example of the kernel boot arguments to attach mtd0
111to UBI and mount volume "rootfs":
112ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
113
114References
115==========
116
117UBIFS documentation and FAQ/HOWTO at the MTD web site:
118http://www.linux-mtd.infradead.org/doc/ubifs.html
119http://www.linux-mtd.infradead.org/faq/ubifs.html
120