1Introduction 2============= 3 4UBIFS file-system stands for UBI File System. UBI stands for "Unsorted 5Block Images". UBIFS is a flash file system, which means it is designed 6to work with flash devices. It is important to understand, that UBIFS 7is completely different to any traditional file-system in Linux, like 8Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems 9which work with MTD devices, not block devices. The other Linux 10file-system of this class is JFFS2. 11 12To make it more clear, here is a small comparison of MTD devices and 13block devices. 14 151 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. 182 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. 223 The whole eraseblock has to be erased before it becomes possible to 23 re-write its contents. Blocks may be just re-written. 244 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. 275 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 32It should be quite obvious why UBIFS is very different to traditional 33file-systems. 34 35UBIFS works on top of UBI. UBI is a separate software layer which may be 36found in drivers/mtd/ubi. UBI is basically a volume management and 37wear-leveling layer. It provides so called UBI volumes which is a higher 38level abstraction than a MTD device. The programming model of UBI devices 39is very similar to MTD devices - they still consist of large eraseblocks, 40they have read/write/erase operations, but UBI devices are devoid of 41limitations like wear and bad blocks (items 4 and 5 in the above list). 42 43In a sense, UBIFS is a next generation of JFFS2 file-system, but it is 44very different and incompatible to JFFS2. The following are the main 45differences. 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 56Similarly to JFFS2, UBIFS supports on-the-flight compression which makes 57it possible to fit quite a lot of data to the flash. 58 59Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts. 60It does not need stuff like fsck.ext2. UBIFS automatically replays its 61journal and recovers from crashes, ensuring that the on-flash data 62structures are consistent. 63 64UBIFS scales logarithmically (most of the data structures it uses are 65trees), so the mount time and memory consumption do not linearly depend 66on the flash size, like in case of JFFS2. This is because UBIFS 67maintains the FS index on the flash media. However, UBIFS depends on 68UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly. 69Nevertheless, UBI/UBIFS scales considerably better than JFFS2. 70 71The authors of UBIFS believe, that it is possible to develop UBI2 which 72would scale logarithmically as well. UBI2 would support the same API as UBI, 73but it would be binary incompatible to UBI. So UBIFS would not need to be 74changed to use UBI2 75 76 77Mount options 78============= 79 80(*) == default. 81 82bulk_read read more in one go to take advantage of flash 83 media that read faster sequentially 84no_bulk_read (*) do not bulk-read 85no_chk_data_crc (*) skip checking of CRCs on data nodes in order to 86 improve read performance. Use this option only 87 if the flash media is highly reliable. The effect 88 of this option is that corruption of the contents 89 of a file can go unnoticed. 90chk_data_crc do not skip checking CRCs on data nodes 91compr=none override default compressor and set it to "none" 92compr=lzo override default compressor and set it to "lzo" 93compr=zlib override default compressor and set it to "zlib" 94 95 96Quick usage instructions 97======================== 98 99The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax, 100where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is 101UBI volume name. 102 103Mount volume 0 on UBI device 0 to /mnt/ubifs: 104$ mount -t ubifs ubi0_0 /mnt/ubifs 105 106Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume 107name): 108$ mount -t ubifs ubi0:rootfs /mnt/ubifs 109 110The following is an example of the kernel boot arguments to attach mtd0 111to UBI and mount volume "rootfs": 112ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs 113 114References 115========== 116 117UBIFS documentation and FAQ/HOWTO at the MTD web site: 118http://www.linux-mtd.infradead.org/doc/ubifs.html 119http://www.linux-mtd.infradead.org/faq/ubifs.html 120
