1 .. SPDX-License-Identifier: GPL-2.0
2
3 ===============
4 UBI File System
5 ===============
6
7 Introduction
8 ============
9
10 UBIFS file-system stands for UBI File System.
11 Block Images". UBIFS is a flash file system, w
12 to work with flash devices. It is important to
13 is completely different to any traditional fil
14 Ext2, XFS, JFS, etc. UBIFS represents a separa
15 which work with MTD devices, not block devices
16 file-system of this class is JFFS2.
17
18 To make it more clear, here is a small compari
19 block devices.
20
21 1 MTD devices represent flash devices and they
22 rather large size, typically about 128KiB. B
23 small blocks, typically 512 bytes.
24 2 MTD devices support 3 main operations - read
25 eraseblock, write to some offset within an e
26 eraseblock. Block devices support 2 main op
27 block and write a whole block.
28 3 The whole eraseblock has to be erased before
29 re-write its contents. Blocks may be just re
30 4 Eraseblocks become worn out after some numbe
31 typically 100K-1G for SLC NAND and NOR flash
32 NAND flashes. Blocks do not have the wear-ou
33 5 Eraseblocks may become bad (only on NAND fla
34 deal with this. Blocks on hard drives typica
35 because hardware has mechanisms to substitut
36 modern LBA disks.
37
38 It should be quite obvious why UBIFS is very d
39 file-systems.
40
41 UBIFS works on top of UBI. UBI is a separate s
42 found in drivers/mtd/ubi. UBI is basically a v
43 wear-leveling layer. It provides so called UBI
44 level abstraction than a MTD device. The progr
45 is very similar to MTD devices - they still co
46 they have read/write/erase operations, but UBI
47 limitations like wear and bad blocks (items 4
48
49 In a sense, UBIFS is a next generation of JFFS
50 very different and incompatible to JFFS2. The
51 differences.
52
53 * JFFS2 works on top of MTD devices, UBIFS dep
54 top of UBI volumes.
55 * JFFS2 does not have on-media index and has t
56 which requires full media scan. UBIFS mainta
57 information on the flash media and does not
58 so it mounts many times faster than JFFS2.
59 * JFFS2 is a write-through file-system, while
60 which makes UBIFS much faster on writes.
61
62 Similarly to JFFS2, UBIFS supports on-the-fly
63 it possible to fit quite a lot of data to the
64
65 Similarly to JFFS2, UBIFS is tolerant of uncle
66 It does not need stuff like fsck.ext2. UBIFS a
67 journal and recovers from crashes, ensuring th
68 structures are consistent.
69
70 UBIFS scales logarithmically (most of the data
71 trees), so the mount time and memory consumpti
72 on the flash size, like in case of JFFS2. This
73 maintains the FS index on the flash media. How
74 UBI, which scales linearly. So overall UBI/UBI
75 Nevertheless, UBI/UBIFS scales considerably be
76
77 The authors of UBIFS believe, that it is possi
78 would scale logarithmically as well. UBI2 woul
79 but it would be binary incompatible to UBI. So
80 changed to use UBI2
81
82
83 Mount options
84 =============
85
86 (*) == default.
87
88 ==================== ======================
89 bulk_read read more in one go to
90 media that read faster
91 no_bulk_read (*) do not bulk-read
92 no_chk_data_crc (*) skip checking of CRCs
93 improve read performan
94 if the flash media is
95 of this option is that
96 of a file can go unnot
97 chk_data_crc do not skip checking C
98 compr=none override default compr
99 compr=lzo override default compr
100 compr=zlib override default compr
101 auth_key= specify the key used f
102 Passing this option ma
103 The passed key must be
104 and must be of type 'l
105 auth_hash_name= The hash algorithm use
106 both hashing and for c
107 include "sha256" or "s
108 ==================== ======================
109
110
111 Quick usage instructions
112 ========================
113
114 The UBI volume to mount is specified using "ub
115 where "X" is UBI device number, "Y" is UBI vol
116 UBI volume name.
117
118 Mount volume 0 on UBI device 0 to /mnt/ubifs::
119
120 $ mount -t ubifs ubi0_0 /mnt/ubifs
121
122 Mount "rootfs" volume of UBI device 0 to /mnt/
123 name)::
124
125 $ mount -t ubifs ubi0:rootfs /mnt/ubifs
126
127 The following is an example of the kernel boot
128 to UBI and mount volume "rootfs":
129 ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
130
131 References
132 ==========
133
134 UBIFS documentation and FAQ/HOWTO at the MTD w
135
136 - http://www.linux-mtd.infradead.org/doc/ubifs
137 - http://www.linux-mtd.infradead.org/faq/ubifs
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