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Merge branch 'linus' into x86/paravirt-spinlocks
Conflicts: arch/x86/kernel/Makefile Signed-off-by: Ingo Molnar <mingo@elte.hu>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,164 @@ | ||
| 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 ckfs.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 | ||
|
|
||
|
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| Mount options | ||
| ============= | ||
|
|
||
| (*) == default. | ||
|
|
||
| norm_unmount (*) commit on unmount; the journal is committed | ||
| when the file-system is unmounted so that the | ||
| next mount does not have to replay the journal | ||
| and it becomes very fast; | ||
| fast_unmount do not commit on unmount; this option makes | ||
| unmount faster, but the next mount slower | ||
| because of the need to replay the journal. | ||
|
|
||
|
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||
| 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 | ||
|
|
||
|
|
||
| Module Parameters for Debugging | ||
| =============================== | ||
|
|
||
| When UBIFS has been compiled with debugging enabled, there are 3 module | ||
| parameters that are available to control aspects of testing and debugging. | ||
| The parameters are unsigned integers where each bit controls an option. | ||
| The parameters are: | ||
|
|
||
| debug_msgs Selects which debug messages to display, as follows: | ||
|
|
||
| Message Type Flag value | ||
|
|
||
| General messages 1 | ||
| Journal messages 2 | ||
| Mount messages 4 | ||
| Commit messages 8 | ||
| LEB search messages 16 | ||
| Budgeting messages 32 | ||
| Garbage collection messages 64 | ||
| Tree Node Cache (TNC) messages 128 | ||
| LEB properties (lprops) messages 256 | ||
| Input/output messages 512 | ||
| Log messages 1024 | ||
| Scan messages 2048 | ||
| Recovery messages 4096 | ||
|
|
||
| debug_chks Selects extra checks that UBIFS can do while running: | ||
|
|
||
| Check Flag value | ||
|
|
||
| General checks 1 | ||
| Check Tree Node Cache (TNC) 2 | ||
| Check indexing tree size 4 | ||
| Check orphan area 8 | ||
| Check old indexing tree 16 | ||
| Check LEB properties (lprops) 32 | ||
| Check leaf nodes and inodes 64 | ||
|
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||
| debug_tsts Selects a mode of testing, as follows: | ||
|
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||
| Test mode Flag value | ||
|
|
||
| Force in-the-gaps method 2 | ||
| Failure mode for recovery testing 4 | ||
|
|
||
| For example, set debug_msgs to 5 to display General messages and Mount | ||
| messages. | ||
|
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||
|
|
||
| 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 |
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