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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 (jacques@solucorp.qc.ca)

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 name
        Permissions and owner
        Links
        Special files (devices, pipe...)
        All is need to be a linux root fs.

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

Mostly...

You mount a DOS partition like this

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

All option 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
happen. This is why all mount options are passed to the Msdos fs driver.

Umsdos use a special DOS file --linux-.--- to store the information
which can't be handle by the normal MsDOS file system. 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 do. Short file name, 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 maintain 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 ...). A ls -l before and after won't show
much difference however. The file which were there are still there. But now
you can do all this:

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

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

What happen if you boot DOS and create files in those promoted directories ?
Umsdos won't notice new files, but will signal removed file (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 insure nice operation. A umsdos.fsck is in the making,
so you will be allowed to managed umsdos partition in the same way
other filesystem are, using the generic fsck front end.

Hope this helps!