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-Introduction
-=============
-
-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
-file-systems.
-
-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
-differences.
-
-* 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 fsck.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.
-
-bulk_read read more in one go to take advantage of flash
- media that read faster sequentially
-no_bulk_read (*) do not bulk-read
-no_chk_data_crc (*) skip checking of CRCs on data nodes in order to
- improve read performance. Use this option only
- if the flash media is highly reliable. The effect
- of this option is that corruption of the contents
- of a file can go unnoticed.
-chk_data_crc do not skip checking CRCs on data nodes
-compr=none override default compressor and set it to "none"
-compr=lzo override default compressor and set it to "lzo"
-compr=zlib override default compressor and set it to "zlib"
-
-
-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
-name):
-$ 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
-
-References
-==========
-
-UBIFS documentation and FAQ/HOWTO at the MTD web site:
-http://www.linux-mtd.infradead.org/doc/ubifs.html
-http://www.linux-mtd.infradead.org/faq/ubifs.html