linux/Documentation/filesystems/ubifs.txt
<<
>>
Prefs
   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
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.