|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
|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 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
|(*) == 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
|$ 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
|UBIFS documentation and FAQ/HOWTO at the MTD web site: