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Linux/Documentation/filesystems/ubifs.txt

  1 Introduction
  2 =============
  3 
  4 UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
  5 Block Images". UBIFS is a flash file system, which means it is designed
  6 to work with flash devices. It is important to understand, that UBIFS
  7 is completely different to any traditional file-system in Linux, like
  8 Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
  9 which work with MTD devices, not block devices. The other Linux
 10 file-system of this class is JFFS2.
 11 
 12 To make it more clear, here is a small comparison of MTD devices and
 13 block devices.
 14 
 15 1 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.
 18 2 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.
 22 3 The whole eraseblock has to be erased before it becomes possible to
 23   re-write its contents. Blocks may be just re-written.
 24 4 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.
 27 5 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 
 32 It should be quite obvious why UBIFS is very different to traditional
 33 file-systems.
 34 
 35 UBIFS works on top of UBI. UBI is a separate software layer which may be
 36 found in drivers/mtd/ubi. UBI is basically a volume management and
 37 wear-leveling layer. It provides so called UBI volumes which is a higher
 38 level abstraction than a MTD device. The programming model of UBI devices
 39 is very similar to MTD devices - they still consist of large eraseblocks,
 40 they have read/write/erase operations, but UBI devices are devoid of
 41 limitations like wear and bad blocks (items 4 and 5 in the above list).
 42 
 43 In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
 44 very different and incompatible to JFFS2. The following are the main
 45 differences.
 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 
 56 Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
 57 it possible to fit quite a lot of data to the flash.
 58 
 59 Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.
 60 It does not need stuff like fsck.ext2. UBIFS automatically replays its
 61 journal and recovers from crashes, ensuring that the on-flash data
 62 structures are consistent.
 63 
 64 UBIFS scales logarithmically (most of the data structures it uses are
 65 trees), so the mount time and memory consumption do not linearly depend
 66 on the flash size, like in case of JFFS2. This is because UBIFS
 67 maintains the FS index on the flash media. However, UBIFS depends on
 68 UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
 69 Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
 70 
 71 The authors of UBIFS believe, that it is possible to develop UBI2 which
 72 would scale logarithmically as well. UBI2 would support the same API as UBI,
 73 but it would be binary incompatible to UBI. So UBIFS would not need to be
 74 changed to use UBI2
 75 
 76 
 77 Mount options
 78 =============
 79 
 80 (*) == default.
 81 
 82 bulk_read               read more in one go to take advantage of flash
 83                         media that read faster sequentially
 84 no_bulk_read (*)        do not bulk-read
 85 no_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.
 90 chk_data_crc            do not skip checking CRCs on data nodes
 91 compr=none              override default compressor and set it to "none"
 92 compr=lzo               override default compressor and set it to "lzo"
 93 compr=zlib              override default compressor and set it to "zlib"
 94 
 95 
 96 Quick usage instructions
 97 ========================
 98 
 99 The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
100 where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
101 UBI volume name.
102 
103 Mount volume 0 on UBI device 0 to /mnt/ubifs:
104 $ mount -t ubifs ubi0_0 /mnt/ubifs
105 
106 Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
107 name):
108 $ mount -t ubifs ubi0:rootfs /mnt/ubifs
109 
110 The following is an example of the kernel boot arguments to attach mtd0
111 to UBI and mount volume "rootfs":
112 ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
113 
114 References
115 ==========
116 
117 UBIFS documentation and FAQ/HOWTO at the MTD web site:
118 http://www.linux-mtd.infradead.org/doc/ubifs.html
119 http://www.linux-mtd.infradead.org/faq/ubifs.html

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