linux/Documentation/filesystems/ubifs.rst
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   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