Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 Introduction 2 ============= 3 4 UBIFS file-system stands for UBI File System. UBI stands for "Unsorted 5 Block Images". UBIFS is a flash file system, which means it is designed 6 to work with flash devices. It is important to understand, that UBIFS 7 is completely different to any traditional file-system in Linux, like 8 Ext2, XFS, JFS, etc. UBIFS represents a separate class of file-systems 9 which work with MTD devices, not block devices. The other Linux 10 file-system of this class is JFFS2. 11 12 To make it more clear, here is a small comparison of MTD devices and 13 block devices. 14 15 1 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. 18 2 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. 22 3 The whole eraseblock has to be erased before it becomes possible to 23 re-write its contents. Blocks may be just re-written. 24 4 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. 27 5 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 32 It should be quite obvious why UBIFS is very different to traditional 33 file-systems. 34 35 UBIFS works on top of UBI. UBI is a separate software layer which may be 36 found in drivers/mtd/ubi. UBI is basically a volume management and 37 wear-leveling layer. It provides so called UBI volumes which is a higher 38 level abstraction than a MTD device. The programming model of UBI devices 39 is very similar to MTD devices - they still consist of large eraseblocks, 40 they have read/write/erase operations, but UBI devices are devoid of 41 limitations like wear and bad blocks (items 4 and 5 in the above list). 42 43 In a sense, UBIFS is a next generation of JFFS2 file-system, but it is 44 very different and incompatible to JFFS2. The following are the main 45 differences. 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 56 Similarly to JFFS2, UBIFS supports on-the-flight compression which makes 57 it possible to fit quite a lot of data to the flash. 58 59 Similarly to JFFS2, UBIFS is tolerant of unclean reboots and power-cuts. 60 It does not need stuff like fsck.ext2. UBIFS automatically replays its 61 journal and recovers from crashes, ensuring that the on-flash data 62 structures are consistent. 63 64 UBIFS scales logarithmically (most of the data structures it uses are 65 trees), so the mount time and memory consumption do not linearly depend 66 on the flash size, like in case of JFFS2. This is because UBIFS 67 maintains the FS index on the flash media. However, UBIFS depends on 68 UBI, which scales linearly. So overall UBI/UBIFS stack scales linearly. 69 Nevertheless, UBI/UBIFS scales considerably better than JFFS2. 70 71 The authors of UBIFS believe, that it is possible to develop UBI2 which 72 would scale logarithmically as well. UBI2 would support the same API as UBI, 73 but it would be binary incompatible to UBI. So UBIFS would not need to be 74 changed to use UBI2 75 76 77 Mount options 78 ============= 79 80 (*) == default. 81 82 bulk_read read more in one go to take advantage of flash 83 media that read faster sequentially 84 no_bulk_read (*) do not bulk-read 85 no_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. 90 chk_data_crc do not skip checking CRCs on data nodes 91 compr=none override default compressor and set it to "none" 92 compr=lzo override default compressor and set it to "lzo" 93 compr=zlib override default compressor and set it to "zlib" 94 95 96 Quick usage instructions 97 ======================== 98 99 The UBI volume to mount is specified using "ubiX_Y" or "ubiX:NAME" syntax, 100 where "X" is UBI device number, "Y" is UBI volume number, and "NAME" is 101 UBI volume name. 102 103 Mount volume 0 on UBI device 0 to /mnt/ubifs: 104 $ mount -t ubifs ubi0_0 /mnt/ubifs 105 106 Mount "rootfs" volume of UBI device 0 to /mnt/ubifs ("rootfs" is volume 107 name): 108 $ mount -t ubifs ubi0:rootfs /mnt/ubifs 109 110 The following is an example of the kernel boot arguments to attach mtd0 111 to UBI and mount volume "rootfs": 112 ubi.mtd=0 root=ubi0:rootfs rootfstype=ubifs 113 114 References 115 ========== 116 117 UBIFS documentation and FAQ/HOWTO at the MTD web site: 118 http://www.linux-mtd.infradead.org/doc/ubifs.html 119 http://www.linux-mtd.infradead.org/faq/ubifs.html