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 ckfs.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 norm_unmount (*)        commit on unmount; the journal is committed
  83                         when the file-system is unmounted so that the
  84                         next mount does not have to replay the journal
  85                         and it becomes very fast;
  86 fast_unmount            do not commit on unmount; this option makes
  87                         unmount faster, but the next mount slower
  88                         because of the need to replay the journal.
  89
  90
  91 Quick usage instructions
  92 ========================
  93
  94 The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
  95 where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
  96 UBI volume name.
  97
  98 Mount volume 0 on UBI device 0 to /mnt/ubifs:
  99 $ mount -t ubifs ubi0_0 /mnt/ubifs
 100
 101 Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
 102 name):
 103 $ mount -t ubifs ubi0:rootfs /mnt/ubifs
 104
 105 The following is an example of the kernel boot arguments to attach mtd0
 106 to UBI and mount volume "rootfs":
 107 ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
 108
 109
 110 Module Parameters for Debugging
 111 ===============================
 112
 113 When UBIFS has been compiled with debugging enabled, there are 3 module
 114 parameters that are available to control aspects of testing and debugging.
 115 The parameters are unsigned integers where each bit controls an option.
 116 The parameters are:
 117
 118 debug_msgs      Selects which debug messages to display, as follows:
 119
 120                 Message Type                            Flag value
 121
 122                 General messages                        1
 123                 Journal messages                        2
 124                 Mount messages                          4
 125                 Commit messages                         8
 126                 LEB search messages                     16
 127                 Budgeting messages                      32
 128                 Garbage collection messages             64
 129                 Tree Node Cache (TNC) messages          128
 130                 LEB properties (lprops) messages        256
 131                 Input/output messages                   512
 132                 Log messages                            1024
 133                 Scan messages                           2048
 134                 Recovery messages                       4096
 135
 136 debug_chks      Selects extra checks that UBIFS can do while running:
 137
 138                 Check                                   Flag value
 139
 140                 General checks                          1
 141                 Check Tree Node Cache (TNC)             2
 142                 Check indexing tree size                4
 143                 Check orphan area                       8
 144                 Check old indexing tree                 16
 145                 Check LEB properties (lprops)           32
 146                 Check leaf nodes and inodes             64
 147
 148 debug_tsts      Selects a mode of testing, as follows:
 149
 150                 Test mode                               Flag value
 151
 152                 Force in-the-gaps method                2
 153                 Failure mode for recovery testing       4
 154
 155 For example, set debug_msgs to 5 to display General messages and Mount
 156 messages.
 157
 158
 159 References
 160 ==========
 161
 162 UBIFS documentation and FAQ/HOWTO at the MTD web site:
 163 http://www.linux-mtd.infradead.org/doc/ubifs.html
 164 http://www.linux-mtd.infradead.org/faq/ubifs.html