Umsdos is a linux file system. It provide an alternative
to the EXT2 file-system. Its main goal is to achieve
easier coexistence with Ms-DOS data by sharing the same
This document explain first how to use Umsdos in different
configuration, and later explain its operation and try to
provide some information letting you decide if it is
a good choice for you (see UMSDOS-WHY-TO at the end).
This document is Copyright (c) 1995 by Jacques Gelinas.
It is released under the terms of the GNU Free Documentation License.
A copy of the license should have been distributed with it, or you can
see a copy at
This document is Copyright (c) 1995, Jacques Gelinas.
It may be distributed under the GNU Free Documentation License.
You should have received a copy with it. If not, you can view it at
The Umsdos project was started in 1992 and made available
to the net in January 1994 as a patch. It was included in the standard
kernel distribution in July, starting with kernel 1.1.36.
Umsdos was early adopted in the Slackware distribution
even before it was officially included in the official kernel.
Umsdos was improved starting at kernel 1.1.60. Its performance
has been dramatically enhanced, especially for writing. Since 1.1.70
(around this), it is stable again.
A major bug was solve in Linux 1.2.2. This bug was causing
some grief to users since the beginning (some file were
silently renamed, giving the sad impression that they were
deleted). Beware that Slackware 2.2 is still shipping
release 1.2.1 of the kernel, so has this bug.
It is available as a patch for kernel 1.0.x. It is built-in
for kernel 1.2. It can be compiled in or load as a module.
Beware that for now, if you intend to load umsdos as a module,
you must also use the Ms-DOS fs as a module. This come from
a limitation in the module system (some symbols are only
export when the drivers is installed as a module).
So far, I think only Slackware does support it. I am surely
wrong, so please send me info to correct this.
The home site for Umsdos is sunsite.unc.edu. Look in
There is quite a lot of documentation about the internal of
Umsdos. It is available both in
HTML and text format
at the same location as the utilities.
As far as I know, the
HTML version is not available online
on any web site. You must down-load it and "UN-tar" it and
read it locally.
Jacques Gelinas email@example.com
With Umsdos, Linux can be installed in a standard
DOS partition. Linux is then installed as a second (or
third) OS in the partition. To avoid name collision (there
is maybe a bin or tmp directory in the drive C: already),
a smart trick: The pseudo-root.
All Linux files are installed in a DOS subdirectory
C: LINUX. The normal
Linux/Unix directory structure goes there. So you
When the Umsdos boot, it probes for the directory
/linux/etc. If it exist, it activates
the pseudo-root mode.
Mostly, the pseudo-root mode switch the root of the partition
C:\\LINUX giving the conventional Unix directory
To this list, it adds a new one called
DOS. This one is
a virtual directory.
- This mode can only be triggered
at boot time. There is no way to activate this by
a mount command.
- This mechanism is purely a different view of a normal
Umsdos file-system. This means that a partition normally
used as a root partition can be normally mounted. There won't
be any pseudo-root effect.
For example, if you boot linux with a maintenance floppy and
mount your normal root partition in
/mnt, you will
find all your linux directory in
/mnt/linux/bin, /mnt/linux/etc and so on.
You can use the same mount option as for the Ms-DOS file system.
The option conv= is questionable on a Umsdos system. I suggest
to avoid it. Mostly the option you may want to look at are
Just remember that Umsdos manage non promoted directory
the same way as the Ms-DOS file system. The option above
will apply globally to all non promoted directory. uid
setup the default owner, gid setup the default group
and umask setup the default permissions.
umssetup was created to provide at run time default ownership
for the root partition. For other Umsdos partition, mount
option may be used or umssetup. Storing mount option in /etc/fstab
is the prefered way for non root partition. Here is an example.
Put this in /etc/rc.d/rc.S.
/sbin/umssetup -u jack -g group -m 0755 /
Using a swap file is generally slower than a swap partition.
It is however much more flexible. You can setup a swap file
in a Umsdos partition the same way you do it for any
other Linux file systems. For example, to setup a
8 megabytes swap file in the root directory:
dd if=/dev/zero bs=1024k count=8 of=/swap
mkswap /swap 8192
Once done, you can put the following line in /etc/fstab
/swap swap swap default
And the swap file will be activated at each boot (There is
generally a "swapon -a" in
The package lodlin15.tgz available from sunsite.unc.edu
/pub/Linux/system/Bootutils. This utility
is particularly suited to boot a Umsdos system. Generally
all you need to do is
C:>loadlinx zimage root=D:
where zimage is a normal kernel image (compressed) simply copied
somewhere in the DOS drive.
D: is the DOS drive where you
have installed Linux.
Booting a Umsdos system from a floppy is not different from
booting a Ext2 system. You need a kernel zImage file properly
initialize to locate your root Umsdos partition. This
is generally achieved using the command
rdev. The following
sequence will initialize a zImage and put it on a floppy.
rdev zImage /dev/hda1
rdev -R zImage 0
dd if=zImage bs=8192 of=/dev/fd0
If this looks confusing, just format a boot-able DOS floppy
and put the following component on it.
and setup the autoexec.bat like this
loadlinx zimage rw root=C:
LILO, the official Linux boot loader can also be used
to boot a Umsdos system. I have no experience with it
though. Since 1.1.60, it should work. Please email if you know
It can be done using any popular DOS tool. There is nothing
particular about file produced by Umsdos. And Umsdos
do not expect anything particular (directory layout, directory
entry sequence, etc...) from the file system under it.
As far as I know, there is no Linux tool to achieve this.
Umsdos rely on the
--linux-.--- which rely on the
DOS directory. Some users may want to experiment
a bit. The utility
udosctl part of the umsdos_progs
basic directory operation (listing, deletion) independently
--linux-.--- and the DOS directory.
Umsdos map Linux files directly to Ms-DOS files.
This is a one for one translation. File content is not manipulated
at all. Umsdos only works on names. For special files (links
and devices for example), it introduces special management.
For each directory, there is a file named
Umsdos can be thought as a general purpose superset
of the Ms-DOS file system of linux. In fact this
capability or flexibility yields much confusion about
Umsdos. Here is why. Try to mount a newly formatted
DOS floppy like this.
mount -t umsdos /dev/fd0 /mnt
And do this,
ls / >/mnt/LONGFILENAME
ls -l /mnt
You will get the following result
-rwxr-xr-x 1 root root 302 Apr 14 23:25 longfile
So far, it seems that the Umsdos file system does not do
much more (in fact nothing at all) than the normal Ms-DOS
file system of Linux.
Pretty unimpressive so far. Here is the trick. Unless promoted
a DOS directory will be managed the same way with Umsdos
than the Ms-DOS file-system will. Umsdos use a special
file in each subdirectory to achieve the translation between
the extended capabilities (long name, ownership, etc...) of
Umsdos and the limitation of the DOS file-system.
This file is invisible to Umsdos users, but visible when
you boot DOS. To avoid cluttering the DOS partition
with those file (
--linux-.---) uselessly, the file is now
optional. If absent, Umsdos behave like Ms-DOS.
When a directory is promoted, any subsequent operation will be
done with the full semantic normally available to Unix and
Linux users. And all subdirectory created afterward will
be silently promoted.
This feature allows you to logically organize your DOS partition
into DOS stuff and Linux stuff. It is important to
understand that those
--linux-.--- file do take some place
(generally 2k per directory). DOS generally use large
cluster (as big as 16k for a 500meg partition), so avoiding
--linux-.--- everywhere can save your day.
A directory can be promoted any time using
It can be used at any time. Promoting a directory do the
- Create a
- Establish a one to one relation between the
and the current content of the directory.
/sbin/umssync maintain an existing
It does not create it from scratch all the time. It simply add
missing entries in it (Files created during a DOS session).
It will also removed files which do not exist anymore in the
DOS directory from the --linux-.---.
its name from that. It put --linux-.--- in sync with
the underlying DOS directory.
It is a good idea to place a call to
at the end of your /etc/rc.d/rc.S if it's not there. The following
command is adequate for most system:
/sbin/umssync -r99 -c -i+ /
-c option prevent
umssync from promoting
directories. It will only update existing
This command is useful if you access Linux directory during
a DOS session. Linux has no efficient way to tell that
a directory has been modified by DOS so Umsdos can't
do a umssync operation as needed.
--linux-.--- file using DOS. You will
Unless you use
umssync on a directory where files have
been added or removed by DOS, you will notice some problems:
- It won't crash the system nor it won't cause major
problems, only annoyance :-)
- Files created by DOS.
- They will be invisible in Linux.
- When trying to create a file with the same name,
you will get an error message stating that the
file already exist.
- This creates more confusion that real problem. It
does not harm the file system.
- Files deleted by DOS won't cause problem. Umsdos
will notice the absence at the first access. A message
will be output (and generally written into
The installation of a Umsdos is not much different
from the installation of an ordinary (Ext2 based) Linux
There are two main differences.
The normal steps for an installation are
- Setting a partition with fdisk and formatting it.
- Mounting it as /mnt relative to our installation
- Copy all packages into
With Umsdos, the step 1 is not required (wasn't it the goal
of Umsdos not to reformat ?).
It is possible to install a Umsdos system just by copying
all packages into
/mnt. This will certainly work. But it
will create a
bunch of subdirectories into your DOS root directory (C:) and
you won't like it. This is the reason all Umsdos installation
use the pseudo-root. And this is the major difference between
a normal Ext2 installation and a Umsdos one: All files
are copied into
/mnt/linux is not an ordinary directory. It has to
be promoted so it will correctly handle Linux long file name
and special files (links, device). The step required to
Even if the setup of
/mnt/linux is pretty simple, there
are many installation package out there who get it wrong. How can ?
The biggest installation problem come from an incompatible
umssync program. Umsdos has been update in
linux 1.1.88 (Can't remember exactly) and a flaw was uncovered
umssync. To avoid confusion in the Linux
community, it was decided to raise the compatibility
level required for all Umsdos tools. Old version
of the tools were simply rejected.
It sounds like many distribution did not update their
umssync utility on the installation disk.
There are still many distribution like this out there. The net result
is that the directory
/mnt/linux is not promoted at all
and will truncate all long file name and will reject all special
It is possible to do a test very early during the installation to
find out if something went wrong. Thanks to the pseudo console
mechanism of Linux, you can do that without leaving the
installation program. Do the following steps:
Alt key at the same time
- login as root.
If this fail, you are trying this too early. A good time
to do this is at the end of the packages selection.
You should see an empty file
TOTO in uppercase. If you
see it in lowercase, something went wrong. Try to do
umssync step again.
umssync can be use over
and over without problem.
If there is no error message, try the
TOTO test again.
TOTO appears fine, then all is OK. Something is strange
in this installation, but you just save it. Continue
Alt-F1 to get back to the installation screen.
If the test fail, the best fix is to get a newer installation
root disk. You can generally fix this root disk by installing
a newer version of
umssync. This is not difficult but
required a working Linux system. You simply have to
mount the root disk floppy and replace the offending
umssync with a new one.
Most Umsdos installation which fail, do this by printing
this strange message. This is not a bug in Umsdos although
the message looks strange. Here are the known causes.
- The most common one
The Slackware installation try to setup a swap
file very early during the installation. To do so, it asks
you to select a partition (dos drive), then mount it and
set the swap file.
When installing a Slackware system, you must setup
the target partition prior to install. This normally
mounts the DOS partition on
/mnt/linux directory and applies
This is where most problems come from. Most user just
forget the "setup target partition" step and go directly
to the rest of the installation. Since
already mounted, this mistake goes unnotice.
This means that
was not created properly (Not promoted). All special files
and links and long names can't be created properly.
- Invalid umssync utility
/mnt/linux was improperly setup-ed. Generally caused
by an improper
umssync utility on the installation
- Old bug in umsdos
There was a bug in Umsdos prior to Linux 1.2.2. The
pseudo-root mode would not activate properly if the
/etc/init was missing.
init is now located
/sbin. You can fix it by getting a newer kernel.
This is recommended because another bug was uncover and fixed
If you can't upgrade, do this
- Boot from you installation disk.
- Login as root.
mount -t umsdos /dev/hdXX /mnt
/dev/hdXX is your DOS partition.
ln -s ../sbin/init init
- Boot your Umsdos normally.
Unfortunatly, the first two (Installation problems) produce a completly
unusable installation. Uninstall it (See next section) and install
One neat thing about Umsdos and its pseudo-root
mechanism, is that you can UN-install it without pain. You
just boot DOS and recursively delete the
directory. That's all. Umsdos requires no special
drivers in the config.sys, nor it creates anything
special outside of the
This can be done from Linux or from DOS.
You just have to copy recursively the
from one drive to the other. After that you will have to
adjust you boot mechanism (generally loadlin command) and
Umsdos can live on any DOS drive. There is no
need to install it on the
C: drive, nor it is important
to have it on the first hard drive. It does not matter at
In fact, one may decide to have several Umsdos installations
on different drive just to do experiments.
How about installing a bunch of Linux systems in no time ?
Umsdos systems are living in a DOS world. You can
take advantage of this if you wish to install Linux easily.
You can install and configure a Umsdos system at your site.
When you are satisfied with the configuration and the different
packages you have selected, you can boot DOS and copy
linux directory to your DOS file
server. Then you go to other DOS station and simply
copy the files on the network drive to the local drive.
That's it. Only adjust the boot script (Loadlinx) and go.
With minimal adjustment (Host name, IP number), anyone will
be able to install a Linux system in a matter of minute.
Interest readers may note that installing Linux systems
by copying running system also works for any other Linux
systems, including Ext2 based one.
One beauty of Linux is that there is no hidden files which
have to be install by magic installation program.
Umsdos has some use even for Ext2 (Native Linux
file-system) users. One common scenario is this:
- Linux being your OS of choice, the Linux
partition start to fill and fill and fill.
- Your DOS partition is collecting dust, being half empty.
- You are suddenly out of space in the Ext2 partition.
- You are still not sure you want to get rid of DOS.
Umsdos may save the day here. You can setup a Linux
directory in the DOS partition and use it without restriction
for Linux usage. For example, say you want to setup a new
"extra" in your
C: drive. And you want this
directory to behave as a normal Linux directory. Do this
(assuming that C: is /dev/hda1).
/sbin/mount -t umsdos /dev/hda1 /c
You must be root to do this.
By setting up
/etc/fstab like this, you will always
have access to the
Explaining how to operate or install a Umsdos system
is not enough. Most people are seeking some advises about
using Umsdos or not.
The goal of Umsdos was to ease the installation of
Linux. An other goal was to ease its UN-installation.
The idea here was to promote the spreading of Linux.
Installing a new OS on a system is always troublesome. OS/2
for one will happily pollute your
C: root with a bunch of
new directories. If you are clever like me, it will also erase
your config.sys and autoexec.bat files :-(
The pseudo-root feature of Umsdos avoid this unwanted
invasion. Linux can be UN-install without side effect.
If you have a small hard drive, Umsdos will allow you
to share disk space between DOS and Linux. A disk
below 300 megs is in my opinion a small disk. This opinion
is based on the size of the different package available today.
One popular word processor may eat as much as 70 megabytes
if you select all features.
If you have a larger drive, you may consider having a dedicated
Linux partition running the Ext2 file-system. Ext2
use a smaller cluster size that DOS (1k in fact) so installing
many small files will eat less space than in a Umsdos
The following point apply to Umsdos compared with Ext2.
- Directory management is faster on Ext2. This come from
the overhead of the double directory structure of
- File access (reading and writing) is probably faster on
Umsdos than Ext2. This come from the simplicity
of the FAT file-system used by DOS.
Beware that this simplicity come with a cost:
- A maximum of around 65,000 files or clusters
per partitions. This also means that a 500
megabytes partition will use cluster 16k large.
In other word, a file containing a single byte
will use 16k of disk storage.
- Everything is controlled by the
at the beginning of the hard drive. The DOS
file-system is probably more fragile because of this.
- No provision to avoid fragmentation of files. A
Umsdos system will generally be used as
a single user workstation. In this case, this does
not matter much. As a multi-user engine, files
will get spread-ed all around the drive, lowering
file access performance.
- Symbolic links are stored in normal file. If you intend
to have a lot of them, you will find that Umsdos
use quite a lot of disk space compared to Ext2.