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