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137 lines
5.7 KiB
137 lines
5.7 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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=============== |
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UBI File System |
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=============== |
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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 fsck.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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==================== ======================================================= |
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bulk_read read more in one go to take advantage of flash |
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media that read faster sequentially |
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no_bulk_read (*) do not bulk-read |
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no_chk_data_crc (*) skip checking of CRCs on data nodes in order to |
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improve read performance. Use this option only |
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if the flash media is highly reliable. The effect |
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of this option is that corruption of the contents |
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of a file can go unnoticed. |
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chk_data_crc do not skip checking CRCs on data nodes |
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compr=none override default compressor and set it to "none" |
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compr=lzo override default compressor and set it to "lzo" |
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compr=zlib override default compressor and set it to "zlib" |
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auth_key= specify the key used for authenticating the filesystem. |
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Passing this option makes authentication mandatory. |
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The passed key must be present in the kernel keyring |
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and must be of type 'logon' |
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auth_hash_name= The hash algorithm used for authentication. Used for |
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both hashing and for creating HMACs. Typical values |
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include "sha256" or "sha512" |
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==================== ======================================================= |
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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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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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