1.. SPDX-License-Identifier: GPL-2.0
2
3===============
4UBI File System
5===============
6
7Introduction
8============
9
10UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
11Block Images". UBIFS is a flash file system, which means it is designed
12to work with flash devices. It is important to understand, that UBIFS
13is completely different to any traditional file-system in Linux, like
14Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
15which work with MTD devices, not block devices. The other Linux
16file-system of this class is JFFS2.
17
18To make it more clear, here is a small comparison of MTD devices and
19block devices.
20
211 MTD devices represent flash devices and they consist of eraseblocks of
22  rather large size, typically about 128KiB. Block devices consist of
23  small blocks, typically 512 bytes.
242 MTD devices support 3 main operations - read from some offset within an
25  eraseblock, write to some offset within an eraseblock, and erase a whole
26  eraseblock. Block  devices support 2 main operations - read a whole
27  block and write a whole block.
283 The whole eraseblock has to be erased before it becomes possible to
29  re-write its contents. Blocks may be just re-written.
304 Eraseblocks become worn out after some number of erase cycles -
31  typically 100K-1G for SLC NAND and NOR flashes, and 1K-10K for MLC
32  NAND flashes. Blocks do not have the wear-out property.
335 Eraseblocks may become bad (only on NAND flashes) and software should
34  deal with this. Blocks on hard drives typically do not become bad,
35  because hardware has mechanisms to substitute bad blocks, at least in
36  modern LBA disks.
37
38It should be quite obvious why UBIFS is very different to traditional
39file-systems.
40
41UBIFS works on top of UBI. UBI is a separate software layer which may be
42found in drivers/mtd/ubi. UBI is basically a volume management and
43wear-leveling layer. It provides so called UBI volumes which is a higher
44level abstraction than a MTD device. The programming model of UBI devices
45is very similar to MTD devices - they still consist of large eraseblocks,
46they have read/write/erase operations, but UBI devices are devoid of
47limitations like wear and bad blocks (items 4 and 5 in the above list).
48
49In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
50very different and incompatible to JFFS2. The following are the main
51differences.
52
53* JFFS2 works on top of MTD devices, UBIFS depends on UBI and works on
54  top of UBI volumes.
55* JFFS2 does not have on-media index and has to build it while mounting,
56  which requires full media scan. UBIFS maintains the FS indexing
57  information on the flash media and does not require full media scan,
58  so it mounts many times faster than JFFS2.
59* JFFS2 is a write-through file-system, while UBIFS supports write-back,
60  which makes UBIFS much faster on writes.
61
62Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
63it possible to fit quite a lot of data to the flash.
64
65Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.
66It does not need stuff like fsck.ext2. UBIFS automatically replays its
67journal and recovers from crashes, ensuring that the on-flash data
68structures are consistent.
69
70UBIFS scales logarithmically (most of the data structures it uses are
71trees), so the mount time and memory consumption do not linearly depend
72on the flash size, like in case of JFFS2. This is because UBIFS
73maintains the FS index on the flash media. However, UBIFS depends on
74UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
75Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
76
77The authors of UBIFS believe, that it is possible to develop UBI2 which
78would scale logarithmically as well. UBI2 would support the same API as UBI,
79but it would be binary incompatible to UBI. So UBIFS would not need to be
80changed to use UBI2
81
82
83Mount options
84=============
85
86(*) == default.
87
88====================	=======================================================
89bulk_read		read more in one go to take advantage of flash
90			media that read faster sequentially
91no_bulk_read (*)	do not bulk-read
92no_chk_data_crc (*)	skip checking of CRCs on data nodes in order to
93			improve read performance. Use this option only
94			if the flash media is highly reliable. The effect
95			of this option is that corruption of the contents
96			of a file can go unnoticed.
97chk_data_crc		do not skip checking CRCs on data nodes
98compr=none              override default compressor and set it to "none"
99compr=lzo               override default compressor and set it to "lzo"
100compr=zlib              override default compressor and set it to "zlib"
101auth_key=		specify the key used for authenticating the filesystem.
102			Passing this option makes authentication mandatory.
103			The passed key must be present in the kernel keyring
104			and must be of type 'logon'
105auth_hash_name=		The hash algorithm used for authentication. Used for
106			both hashing and for creating HMACs. Typical values
107			include "sha256" or "sha512"
108====================	=======================================================
109
110
111Quick usage instructions
112========================
113
114The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
115where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
116UBI volume name.
117
118Mount volume 0 on UBI device 0 to /mnt/ubifs::
119
120    $ mount -t ubifs ubi0_0 /mnt/ubifs
121
122Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
123name)::
124
125    $ mount -t ubifs ubi0:rootfs /mnt/ubifs
126
127The following is an example of the kernel boot arguments to attach mtd0
128to UBI and mount volume "rootfs":
129ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
130
131References
132==========
133
134UBIFS documentation and FAQ/HOWTO at the MTD web site:
135
136- http://www.linux-mtd.infradead.org/doc/ubifs.html
137- http://www.linux-mtd.infradead.org/faq/ubifs.html
138