umsdos.txt

ARM 嵌入式 系统 设计与实例开发 实验教材 二源码

Firstly, let me say that UMSDOS is going through some major code changes,
and has some KNOWN BUGS (and quite a few unknown :-). Please read
fs/umsdos/README-WIP.txt for more information on current status. Thanks.

----------------------------------------------------------------------------
Very short explanation for the impatient!

Umsdos is a file system driver that run on top the MSDOS fs driver.
It is written by Jacques Gelinas ([e-mail])
and is currently maintained by Matija Nalis ([e-mail])

Umsdos is not a file system per se, but a twist to make a boring
one into a useful one.

It gives you:

	long file names
	Permissions and owners
	Links
	Special files (devices, pipes...)
	All that is needed to be a linux root fs.

There is plenty of documentation on it in the source. A formatted document
made from those comments is available from
sunsite.unc.edu:/pub/Linux/system/Filesystems/umsdos.

You mount a DOS partition like this:

mount -t umsdos /dev/hda3 /mnt
         ^
---------|

All options are passed to the msdos drivers. Option like uid,gid etc are
given to msdos.

The default behavior of Umsdos is to do the same thing as the msdos driver
mostly passing commands to it without much processing. Again, this is
the default. After doing the mount on a DOS partition, nothing special
happens. This is why all mount options are passed to the msdos fs driver.

Umsdos uses a special DOS file --linux-.--- to store the information
which can't be handled by the normal MS-DOS filesystem. This is the trick.

--linux-.--- is optional. There is one per directory.

**** If --linux-.--- is missing, then Umsdos process the directory the
     same way the msdos driver does. Short file names, no goodies, default
     owner and permissions. So each directory may have or not this
     --linux-.---

Now, how to get those --linux-.---.

\begin joke_section

	Well send me a directory content
	and I will send you one customised for you.
	$5 per directory. Add any applicable taxes.
\end joke_section

A utility umssync creates those. The kernel maintains them. It is available
from the same directory above (sunsite) in the file umsdos_progs-0.7.tar.gz.
A compiled version is available in umsdos_progs-0.7.bin.tar.gz.

So in our example, after mounting mnt, we do

	umssync .

This will promote this directory (a recursive option is available) to full
umsdos capabilities (long name, etc.).  However, an "ls -l" before and after
won't show much difference.  The files which were there are still there, but
now you can do all this:

	chmod 644 *
	chown you.your_group *
	ls >THIS_IS.A.VERY.LONG.NAME
	ln -s toto tata
	ls -l

Once a directory is promoted, all subdirectories created will inherit that
promotion.

What happens if you boot DOS and create files in those promoted directories ?
Umsdos won't notice new files, but will signal removed files (it won't crash).
Using umssync in /etc/rc will make sure the DOS directory is in sync with
the --linux-.---.

It is a good idea to put the following command in your RC file just
after the "mount -a":

	mount -a
	/sbin/umssync -i+ -c+ -r99 /umsdos_mount_point

	(You put one for each umsdos mount point in the fstab)

This will ensure nice operation.  A umsdos.fsck is in the making,
so you will be allowed to manage umsdos partitions in the same way
other filesystems are, using the generic fsck front end.

Hope this helps!