diff options
author | Yunhong Jiang <yunhong.jiang@intel.com> | 2015-08-04 12:17:53 -0700 |
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committer | Yunhong Jiang <yunhong.jiang@intel.com> | 2015-08-04 15:44:42 -0700 |
commit | 9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00 (patch) | |
tree | 1c9cafbcd35f783a87880a10f85d1a060db1a563 /kernel/Documentation/filesystems/ubifs.txt | |
parent | 98260f3884f4a202f9ca5eabed40b1354c489b29 (diff) |
Add the rt linux 4.1.3-rt3 as base
Import the rt linux 4.1.3-rt3 as OPNFV kvm base.
It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and
the base is:
commit 0917f823c59692d751951bf5ea699a2d1e2f26a2
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Sat Jul 25 12:13:34 2015 +0200
Prepare v4.1.3-rt3
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
We lose all the git history this way and it's not good. We
should apply another opnfv project repo in future.
Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Diffstat (limited to 'kernel/Documentation/filesystems/ubifs.txt')
-rw-r--r-- | kernel/Documentation/filesystems/ubifs.txt | 119 |
1 files changed, 119 insertions, 0 deletions
diff --git a/kernel/Documentation/filesystems/ubifs.txt b/kernel/Documentation/filesystems/ubifs.txt new file mode 100644 index 000000000..a0a61d2f3 --- /dev/null +++ b/kernel/Documentation/filesystems/ubifs.txt @@ -0,0 +1,119 @@ +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 |