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.
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.
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
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
debug_tsts Selects a mode of testing, as follows:
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.
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
