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Subject[PATCH take 3 04/29] UBIFS: add brief documentation
Signed-off-by: Artem Bityutskiy <>
Signed-off-by: Adrian Hunter <>
Documentation/filesystems/ubifs.txt | 163 +++++++++++++++++++++++++++++++++++
1 files changed, 163 insertions(+), 0 deletions(-)
diff --git a/Documentation/filesystems/ubifs.txt b/Documentation/filesystems/ubifs.txt
new file mode 100644
index 0000000..ab653b4
--- /dev/null
+++ b/Documentation/filesystems/ubifs.txt
@@ -0,0 +1,163 @@
+UBIFS file-system stands for UBI File System. UBI stands for "Unsorted
+Block Images". UBIFS is a flash file system, which means it is designed
+to work with flash devices. It is important to understand, that UBIFS
+is completely different to any traditional file-system in Linux, like
+Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems
+which work with MTD devices, not block devices. The other Linux
+file-system of this class is JFFS2.
+To make it more clear, here is a small comparison of MTD devices and
+block devices.
+1 MTD devices represent flash devices and they consist of eraseblocks of
+ rather large size, typically about 128KiB. Block devices consist of
+ small blocks, typically 512 bytes.
+2 MTD devices support 3 main operations - read from some offset within an
+ eraseblock, write to some offset within an eraseblock, and erase a whole
+ eraseblock. Block devices support 2 main operations - read a whole
+ block and write a whole block.
+3 The whole eraseblock has to be erased before it becomes possible to
+ re-write its contents. Blocks may be just re-written.
+4 Eraseblocks become worn out after some number of erase cycles -
+ typically 100K-1G for SLC NAND and NOR flashes, and 1K-10K for MLC
+ NAND flashes. Blocks do not have the wear-out property.
+5 Eraseblocks may become bad (only on NAND flashes) and software should
+ deal with this. Blocks on hard drives typically do not become bad,
+ because hardware has mechanisms to substitute bad blocks, at least in
+ modern LBA disks.
+It should be quite obvious why UBIFS is very different to traditional
+UBIFS works on top of UBI. UBI is a separate software layer which may be
+found in drivers/mtd/ubi. UBI is basically a volume management and
+wear-leveling layer. It provides so called UBI volumes which is a higher
+level abstraction than a MTD device. The programming model of UBI devices
+is very similar to MTD devices - they still consist of large eraseblocks,
+they have read/write/erase operations, but UBI devices are devoid of
+limitations like wear and bad blocks (items 4 and 5 in the above list).
+In a sense, UBIFS is a next generation of JFFS2 file-system, but it is
+very different and incompatible to JFFS2. The following are the main
+* JFFS2 works on top of MTD devices, UBIFS depends on UBI and works on
+ top of UBI volumes.
+* JFFS2 does not have on-media index and has to build it while mounting,
+ which requires full media scan. UBIFS maintains the FS indexing
+ information on the flash media and does not require full media scan,
+ so it mounts many times faster than JFFS2.
+* JFFS2 is a write-through file-system, while UBIFS supports write-back,
+ which makes UBIFS much faster on writes.
+Similarly to JFFS2, UBIFS supports on-the-flight compression which makes
+it possible to fit quite a lot of data to the flash.
+Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts.
+It does not need stuff like ckfs.ext2. UBIFS automatically replays its
+journal and recovers from crashes, ensuring that the on-flash data
+structures are consistent.
+UBIFS scales logarithmically (most of the data structures it uses are
+trees), so the mount time and memory consumption do not linearly depend
+on the flash size, like in case of JFFS2. This is because UBIFS
+maintains the FS index on the flash media. However, UBIFS depends on
+UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly.
+Nevertheless, UBI/UBIFS scales considerably better than JFFS2.
+The authors of UBIFS believe, that it is possible to develop UBI2 which
+would scale logarithmically as well. UBI2 would support the same API as UBI,
+but it would be binary incompatible to UBI. So UBIFS would not need to be
+changed to use UBI2
+Mount options
+(*) == default.
+norm_unmount (*) commit on unmount; the journal is committed
+ when the file-system is unmounted so that the
+ next mount does not have to replay the journal
+ and it becomes very fast;
+fast_unmount do not commit on unmount; this option makes
+ unmount faster, but the next mount slower
+ because of the need to replay the journal.
+Quick usage instructions
+The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax,
+where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is
+UBI volume name.
+Mount volume 0 on UBI device 0 to /mnt/ubifs:
+$ mount -t ubifs ubi0_0 /mnt/ubifs
+Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume
+$ mount -t ubifs ubi0:rootfs /mnt/ubifs
+The following is an example of the kernel boot arguments to attach mtd0
+to UBI and mount volume "rootfs":
+ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs
+Module Parameters for Debugging
+When UBIFS has been compiled with debugging enabled, there are 3 module
+parameters that are available to control aspects of testing and debugging.
+The parameters are unsigned integers where each bit controls an option.
+The parameters are:
+debug_msgs Selects which debug messages to display, as follows:
+ Message Type Flag value
+ General messages 1
+ Journal messages 2
+ Mount messages 4
+ Commit messages 8
+ LEB search messages 16
+ Budgeting messages 32
+ Garbage collection messages 64
+ Tree Node Cache (TNC) messages 128
+ LEB properties (lprops) messages 256
+ Input/output messages 512
+ Log messages 1024
+ Scan messages 2048
+ Recovery messages 4096
+debug_chks Selects extra checks that UBIFS can do while running:
+ Check Flag value
+ General checks 1
+ Check Tree Node Cache (TNC) 2
+ Check indexing tree size 4
+ Check orphan area 8
+ Check old indexing tree 16
+ Check LEB properties (lprops) 32
+debug_tsts Selects a mode of testing, as follows:
+ Test mode Flag value
+ Force in-the-gaps method 2
+ Failure mode for recovery testing 4
+For example, set debug_msgs to 5 to display General messages and Mount
+UBIFS documentation and FAQ/HOWTO at the MTD web site:

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