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| HowTo Linux Zone | Linux Zone Home | E-Mail Me | UMSDOS HOW-TO Jacques Gelinas, jacques@solucorp.qc.ca
v1.1, 13 November 1995
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 partition. 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).
1. UMSDOS: Where is it ?
1.1. History
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.
1.2. Availability
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).
1.3. Distribution supporting it
So far, I think only Slackware does support it. I am surely wrong, so
please send me info to correct this.
1.4. Home site
The home site for Umsdos is sunsite.unc.edu. Look in the directory
/pub/Linux/system/Filesystems/umsdos.
1.5. Technical documentation
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.
1.6. Who wrote it
Jacques Gelinas jacques@solucorp.qc.ca
2. Umsdos as your root partition
2.1. The pseudo-root concept.
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), Umsdos use a smart trick: The pseudo-root.
All Linux files are installed in a DOS subdirectory called linux,
generally C: LINUX. The normal Linux/Unix directory structure goes
there. So you get
·
C:\LINUX\BIN
·
C:\LINUX\ETC
·
C:\LINUX\LIB
·
C:\LINUX\ROOT
·
C:\LINUX\SBIN
·
C:\LINUX\TMP
·
C:\LINUX\USR
·
C:\LINUX\VAR
When the Umsdos boot, it probes for the directory linux and then
/linux/etc. If it exist, it activates the pseudo-root mode.
Mostly, the pseudo-root mode switch the root of the partition to
C:\LINUX giving the conventional Unix directory layout
·
/bin
·
/etc
·
/lib
·
/root
·
/sbin
·
/tmp
·
/usr
·
/var
To this list, it adds a new one called DOS. This one is a virtual
directory.
2.2. Things to know about the pseudo-root
· 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.
3. Different topics about the operation of Umsdos
3.1. Mount option
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
· uid=
· gid=
· umask=
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.
3.2. How to set defaults for the root
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 /
3.3. To swap or not to swap
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
sync
swapon /swap
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 /etc/rc.d/rc.S).
4. How to boot a Umsdos system
4.1. Loadlin
The package lodlin15.tgz available from sunsite.unc.edu in
/pub/Linux/system/Bootutils. This utility is particularly suited to
boot a Umsdos system. Generally all you need to do is
Boot DOS
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.
4.2. From a floppy
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.
· loadlin.exe
· loadlinx.exe
· zimage
and setup the autoexec.bat like this
loadlinx zimage rw root=C:
4.3. LILO
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 something.
4.4. How to defragment a Umsdos partition
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.
4.5. Advance tricks
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 package (containing umssync and umssetup) allows basic
directory operation (listing, deletion) independently on the
--linux-.--- and the DOS directory.
5. Basic principle
5.1. Introduction
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 --linux-.---.
5.2. Umsdos can replace the Ms-DOS file-system.
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.
???
5.3. Directory promotion
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 putting --linux-.--- everywhere can save your day.
5.4. How to promote: /sbin/umssync
A directory can be promoted any time using /sbin/umssync. It can be
used at any time. Promoting a directory do the following operation
· Create a --linux-.---.
· Establish a one to one relation between the --linux-.--- and the
current content of the directory.
/sbin/umssync maintain an existing --linux-.--- file. 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-.---.
umssync gets its name from that. It put --linux-.--- in sync with the
underlying DOS directory.
5.5. Using /sbin/umssync at boot time
It is a good idea to place a call to /sbin/umssync 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+ /
The -c option prevent umssync from promoting directories. It will only
update existing --linux-.---.
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.
5.6. How to UN-promote
Remove the --linux-.--- file using DOS. You will be sorry.
5.7. What about files created during a DOS session ?
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 /var/adm/syslog).
6. Installation/UN-installation and some tricks
The installation of a Umsdos is not much different from the
installation of an ordinary (Ext2 based) Linux system.
There are two main differences.
6.1. The pseudo-root /mnt/linux
The normal steps for an installation are
1. Setting a partition with fdisk and formatting it.
2. Mounting it as /mnt relative to our installation root disk.
3. Copy all packages into /mnt.
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.
6.2. Preparing /mnt/linux
/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 setup /mnt/linux are:
1. mkdir /mnt/linux
2. umssync /mnt/linux
That's it!
6.3. Making sure /mnt/linux is correctly setup
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 in 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 file.
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:
1. Press Alt-F2 (Alt key at the same time as the F2 key).
2. login as root.
3. cd /mnt/linux
If this fail, you are trying this too early. A good time to do this
is at the end of the packages selection.
4. >TOTO
5. ls -l
You should see an empty file TOTO in uppercase. If you see it in
lowercase, something went wrong. Try to do the umssync step again.
umssync can be use over and over without problem.
umssync .
If there is no error message, try the TOTO test again. If TOTO
appears fine, then all is OK. Something is strange in this
installation, but you just save it. Continue
6. Press 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.
6.4. Oops releasing pseudo root ...
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, creates the /mnt/linux directory and applies umssync on
it.
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 /mnt is already mounted, this mistake goes
unnotice. This means that /mnt/linux 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 root disk.
· 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 file /etc/init was missing.
init is now located in /sbin. You can fix it by getting a newer
kernel. This is recommended because another bug was uncover and
fixed in 1.2.2.
If you can't upgrade, do this
1. Boot from you installation disk.
2. Login as root.
3. mount -t umsdos /dev/hdXX /mnt
where /dev/hdXX is your DOS partition.
4. cd /mnt/linux/etc
5. ln -s ../sbin/init init
6. cd /
7. Ctrl-Alt-Del
8. Boot your Umsdos normally.
Unfortunatly, the first two (Installation problems) produce a
completly unusable installation. Uninstall it (See next section) and
install again.
6.5. How to UN-install a Umsdos system
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 linux directory. That's all. Umsdos requires no special
drivers in the config.sys, nor it creates anything special outside of
the linux directory.
6.6. Moving a Umsdos system to another DOS drive
This can be done from Linux or from DOS. You just have to copy
recursively the linux directory from one drive to the other. After
that you will have to adjust you boot mechanism (generally loadlin
command) and the /etc/fstab file.
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 all.
In fact, one may decide to have several Umsdos installations on
different drive just to do experiments.
6.7. About installing 50 Umsdos systems.
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 the complete 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.
7. Setting a Linux section in a DOS partition
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 directory named "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).
mkdir /c
/sbin/mount -t umsdos /dev/hda1 /c
mkdir /c/extra
umssync /c/extra
You must be root to do this.
By setting up /etc/fstab like this, you will always have access to the
/c/extra directory.
8. UMSDOS-WHY-TO
Explaining how to operate or install a Umsdos system is not enough.
Most people are seeking some advises about using Umsdos or not.
8.1. The goal of Umsdos
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.
8.2. Who needs it
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 partition.
8.3. Performance issue
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 Umsdos.
· 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 FAT located 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.
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