Linux Swap Space Mini-HOWTO

Linux Swap Space Mini-HOWTO
Rahul U. Joshi, [email protected] <mailto:jurahul@hot�
mail.com> http://jurahul.freeservers.com
<http://jurahul.freeservers.com>
v1.42, 18 January 2000

This Linux Mini-HOWTO describes how to share your Linux swap partition
with Windows.
______________________________________________________________________

Table of Contents

1. Introduction

2. What you need

3. Recent Changes and versions

4. Administrative

4.1 Copyright
4.2 Disclaimer

5. Before you begin

6. The Procedure

6.1 Turn off swapping and create a DOS partition
6.2 Tell Windows the location of the new swap file
6.2.1 (BF
6.2.2 (BF
6.3 Back up the Total Special Sectors
6.4 Modify the initialization and shutdown scripts to handle our new configuration
6.5 Reenable swapping

7. A couple of notes

8. What are we exactly doing ?

9. The swapinit.sh script

10. The swaphalt.sh script

11. The msinfo.sh script

12. The original msinfo file

13. Acknowledgements, feedback and dedication

______________________________________________________________________

1. Introduction

Many people use both Linux and MS-Windows. The ability to do so is an
important part of "the Linux revolution"; i.e. letting people
experiment with (and get hooked on) Linux while still being able to
run their off-the-shelf software. Since both Linux and MS-Windows use
virtual memory with swap to disk, a frequently occurring question in
comp.os.linux.setup is how to share swap spaces, in order to reduce
the amount of disk space needed.

There are several methods for sharing swap spaces, the one described
in this document is probably the most complicated one but is the only
one I have encountered that allows maximum performance for both
environments without the risk of trashing a disk partition. If you are
using DOS, then many DOS applications manage their own swap files. You
can also share the Linux swap partition with these applications.

2. What you need

This procedure have a few requirements that need to be filled. I
strongly recommend that you fill these requirements *anyway*, as there
are several problems with older versions.

1. MS-DOS 5.0 or newer and MS-Windows 3.1/3.11 OR MS-Windows 95/98

2. A shutdown/init that knows to run a file on shutdown. (The
SysVinit-2.50 package can do this, for example. SysVinit-2.50 is
available from sunsite.unc.edu in /pub/Linux/system/Daemons.
Almost all current distributions use this init package.)

3. Recent Changes and versions

� Version 1.3 -- Written by H. Peter Anvin, and was not maintained
from 15 Jan 1998.

� Version 1.4 -- Based on v1.3, but added more details. Currently
maintained by Rahul U. Joshi

� Version 1.42 -- Deleted the recovery section for v1.4 users an v1.4
never actually appeared on the LDP website.

Modifications by Rahul U. Joshi

Date: 26 Nov 1999

� Added Windows 95/98 specific details.

� Added Red Hat Linux specific details.

� Modified the msinfo program.

� Added sections 5 to 12.

� Completely revamped the "Procedure" section to make it more
understandable.

� Split up the swap initialization and halt into two shell scripts.

� Modified the scripts to handle signature verification more than
once.

� Added details about new type swap areas.

� Added more reliable schemes.

4. Administrative

4.1. Copyright

Unless otherwise stated, Linux HOWTO documents are copyrighted by
their respective authors. Linux HOWTO documents may be reproduced and
distributed in whole or in part, in any medium physical or electronic,
as long as this copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the author would
like to be notified of any such distributions.

All translations, derivative works, or aggregate works incorporating
any Linux HOWTO documents must be covered under this copyright notice.
That is, you may not produce a derivative work from a HOWTO and impose
additional restrictions on its distribution. Exceptions to these rules
may be granted under certain conditions; please contact the Linux
HOWTO coordinator at the address given below.

In short, we wish to promote dissemination of this information through
as many channels as possible. However, we do wish to retain copyright
on the HOWTO documents, and would like to be notified of any plans to
redistribute the HOWTOs.

If you have questions, please contact Tim Bynum, the Linux HOWTO
coordinator, at [email protected] <mailto:linux-
[email protected]> via email.

4.2. Disclaimer

Although I have tried my best to bring out various facts, the author
is not responsible for any damage due to the actions taken based upon
the information contained in this document. It is impossible to test
the things under all the configurations, so probably some of the hints
given in this document may be buggy and may not work on your system.
In case you trace the bug, let me know it first !

This document is provided ``as is''. I put great effort into writing
it as accurately as I could, but you use the information contained in
it at your own risk. In no event shall I be liable for any damages
resulting from the use of this work.

In case you are planning to include this HOWTO on some distribution
medium or in print, I will like to have an acknowledgement e-mail
(just for my record). In that case, I may also be able to send you the
latest version of the document.

5. Before you begin

This is not a step by step HOWTO. The reader is expected to first read
the entire document so that he (or she) becomes aware of the possible
pitfalls and performance bottlenecks. After having a fair idea as to
what steps are to be taken, he (or she) can then follow the next
section in a step by step manner. Since you will be playing as a root
and that too with the system initialization files, be alert as to what
you are doing. Write in those files comments for those sections that
you added, and when you want to remove a section, do not delete it.
Instead just comment it out. (In shell scripts comments begin with a
pound(#) sign). In case things go wrong, you may restore your original
configuration by removing the changes you did. Although you are not
required to know shell scripting since I have given all those that
would be needed, some knowledge may be advantageous both for
understanding as well for debugging.

6. The Procedure

NOTE: This procedure has been written keeping in mind RedHat Linux 6.0
Although this procedure in general is applicable for all Linux
distributions, the details may vary. You are welcome to add the
details for your distribution. Many users will already have a swap
partition devoted to Linux. I assume you have one.

6.1. Turn off swapping and create a DOS partition

�

Boot Linux. Turn off swapping by editing the /etc/fstab file and
commenting the line that describes your swap partition.

�

Using fdisk under Linux, delete the swap partition in order to
create free space on the disk.

�

Boot DOS. Create a DOS partition (using FDISK) the size = the size
swap space you want. If you cannot create the partition (probably
because DOS FDISK says that all the space in extended partition is
allocated to the logical drive), you can use fdisk or cfdisk under
Linux to create the partition. You can even just change the type of
your swap partition from 82h (Linux Swap) to 06h (FAT16). In that
case you don't need to delete the swap partition, just change the
type.

�

Once you have created the partition, Boot DOS if you are not
already in DOS. DOS will be assigned a drive letter to your new
partition. Use that drive letter instead of X whenever these
instructions lists a command like "LABEL X:" or "COPY FOO
X:DUMMY.DAT"

�

Format this partition using the DOS FORMAT command.

C:\>FORMAT X:

�

Set the volume label on this partition to "SWAP SPACE" using the
DOS LABEL command. Verify it by the DIR command. Please do this
as a separate step. Some versions of FORMAT do not seem to put the
volume label in the boot sector as it should. [Note: some people
has written me saying the volume label is stored in the root
directory. Yes, but at least since DOS 5.0 it has also been in the
boot sector.]

C:\>LABEL X:
C:\>DIR X:

�

At this stage, you have a DOS partition ready for use for swapping.

6.2. Tell Windows the location of the new swap file

6.2.1. For Windows 3.1 users

�

Start Windows. Go to the Control Panel, select "386 Enhanced".
Select "Virtual Memory" and create a Windows Permanent swap file on
drive X: of maximum size (Windows will tell you the maximum size).
Windows may complain saying it will not use a swap file that big.
Ignore the message and create the file anyway.

�

Exit Windows.

6.2.2. For Windows 95/98 users

�

Start Windows. Go to the Control Panel (Start | Settings | Control
panel). Double click the System icon. In the resulting System
Properties dialog box, select the Performance tab. Now click the
Virtual Memory button. This will bring up the Virtual Memory dialog
box. Select "Let me specify my own virtual memory settings". Select
the drive you intend to share as swap space with Linux (X:). Keep
the Maximum and Minimum values as selected by Windows. Click OK.
Windows will prompt you to restart your computer. Do it.

�

When you restart Windows, you will see that the win386.swp file has
been created on the X: drive. In addition, there is also the
Recycled folder. Since you don't intend to store any other files
on this drive and also a single file on this partition will be
convienent, tell Windows not to maintain Recycle Bin for this (X:)
drive. On the Desktop, left click the Recycle Bin icon, and in the
resulting pop-up menu, click Properties. This will bring up the
Recycle Bin Properties dialog box. Click the Global tab and select
"Configure drives independently". Now click the tab for X: drive
(which will have the label SWAP SPACE). In this dialog box, move
the slider for "Maximum size of Recycle Bin" to 0 percent. The
"Space Reserved" entry should now read as 0 bytes. Click OK.
Restart your computer.

�

Now that you are not maintaining the Recycle Bin for drive X:,
ideally the RECYCLED folder from X: must be deleted by Windows. Not
only Windows doesn't does that, but also does not allow you to
delete the folder. But as I said earlier, this folder is now
unnecessary and we want drive X: to contain only the win386.swp
file. For that purpose, first start your computer in MS-DOS mode.
Select Start | Shut Down | Restart your computer in MS-DOS mode, or
if you are booting Windows, press F8 at the start, and then from
the startup menu, select Command Prompt only. Once you are in MS-
DOS mode, type the following commands:

C:\>X:
X:\>ATTRIB -R -H -S RECYCLED
X:\>DELTREE RECYCLED

Now you have drive X: containing only win386.swp. You can verify this
using the various options for the dircommand. This does not mean that
the RECYCLED folder will not be there on drive X:. When Windows boots
up and finds that there is no RECYCLED folder on X:, it will create it
automatically. However, when we will be restoring X: from Linux, it
will simply contain win386.swp file. If anyone is aware of a method to
prevent Windows from creating the RECYCLED folder, please e-mail me.

�
Disk Cleanup troubles you ?

If you have Disk Cleanup installed, then ocassionally Windows comes
out with a "Low disk space on drive X:". Since we are using the
entire X: drive for swapping, there is no need to have any free
space on X:. To prevent this dialog box from appearing, start Disk
Cleanup (point to Start | Programs | Accessories | System Tools |
Disk Cleanup). In the resulting dialog box, select the X: drive and
click OK. The "Disk Cleanup for drive X:" dialog box appears. Click
the "Settings" tab and uncheck the "If this drive runs low on disk
... blah blah blah" check box.

>From this stage onwards, Windows will assume that it's swap file is
on drive X:. So the drive X: must be intact each time you boot
Windows. If you are using some additional system utilities like Norton
Utilities for Windows 95, then you should probably consult the online
help or the manuals to keep them informed of the changes in the
settings, otherwise they may come out with an error message.

6.3. Back up the Total Special Sectors

�

Boot Linux, then log in as root.

�

Use the fdisk command to find the name of the partition and its
size in blocks. Create a symbolic link from /dev/winswap to this
partition. If the partition is hda7, then type:

# ln -s /dev/hda7 /dev/winswap

[NOTE TO PURISTS: Please use a symlink. The name of this partition is
going to go into several configuration files and inconsistencies could
be fatal.]

�

Find the "Total Special Sectors" for the swap drive. These are
nothing but the total number of sectors required for the boot
sector, FAT and root directory. There are a number of ways to find
this number. Simplest copy the ``msinfo.sh'' file using your
favourite editor. Give the following commands at the shell prompt

# cp Swap-Space-HOWTO msinfo.sh
# chmod +x msinfo.sh

(Much easier if you use one of the editors with KDE or GNOME)

Now, open the msinfo.sh file in the editor and delete everything
except the msinfo.sh file in section 8. Now run this file as

# ./msinfo.sh /dev/winswap

The program will print the some information about the swap partition.
Take note at the number saying "Total special sectors", and verify
that the volume label says "SWAP SPACE". If it does not, reboot DOS
and re-do the LABEL command.

�

Note: You can also find the "Total Special Sectors" using tools
that can display the Boot sector like the Norton Disk Editor or
minfo from mtools under Linux. For Norton Disk Editor, select
"Boot Sector" from the Object menu. Note down the following entries
:

Reserved Sectors at beginning : r
FAT Copies : f
Sectors per FAT : s
Root directory entries : r

Now the "Total Special Sectors" is given by

T = r + (s * f) + (d / 16)

However, using the above program is more convienent.

Let us denote these Total Special Sectors by XXX.

�

[Optional step] Windows may occasionally leave some space on the
partition, even if it is told not to. Don't attempt to use this
space, since it will be erased any time you run Linux. If you want
to avoid accidentally using it (and lose data), you can create a
dummy file that fills that space by using the following commands:

mkdir /mnt
mount -t msdos /dev/winswap /mnt
dd if=/dev/zero of=/mnt/dummy.fil
umount /mnt

The dd command will report "No space left on device". This is exactly
what you want.

�

Type on the shell prompt:

# dd if=/dev/winswap bs=512 count=XXX | gzip -9 > /etc/winswap.gz
^^^

... where XXX is replaced with the "Total special sectors" number.

Here we are saving the Total Special Sectors in a compressed form in
the file /etc/winswap.gz

6.4. Modify the initialization and shutdown scripts to handle our new
configuration

�

Now, we will modify our initalization scripts so that swap space
will be create on the partition each time Linux starts and the
DOS/Windows special sectors will be restored each time Linux shut
downs.

�

Instead of placing the command for handling the details directly in
the initialization scripts, we will prepare two separate files
called swapinit.sh and swaphalt.sh. One sample of these files that
should work under Red Hat Linux is given in section at the end. You
can choose to place these files under any directory, preferably
/etc/rc.d/init.d. Create the two files in this directory and copy
into then the scripts given in sections at the end. One simple way
is to copy this HOWTO in these files as,

# cp Swap-Space-HOWTO /etc/rc.d/init.d/swapinit.sh
# cp Swap-Space-HOWTO /etc/rc.d/init.d/swaphalt.sh

and then using your favourite editor, delete all execpt the required
part. Now make these file "executable" by giving the following
commands

# chmod +x swapinit.sh
# chmod +x swaphalt.sh

�

Run fdisk and find the size of the swap partition in blocks. In
both the above files (swap????.sh) replace the YYYYY by this
number.

�

Check the name of the shutdown file. For SysVinit this is the file
listed in the following line of /etc/inittab; add it if you don't
have it.

# Runlevel 0 means shut down the system
l0:0:wait:/etc/brc

(Under Red Hat Linux, this file probably will be
/etc/rc.d/init.d/halt) If you are not in a position to find it out,
browse through the /etc directory and find the initialization file
that contains the "swapon -a" command. Only this much for now !

For the remainder of this file, I will assume the filename was
/etc/rc.d/init.d/halt (halt for short).

�

Check the name of your system initialization file. For SysVinit,
this is the file listed in the following line of /etc/inittab.

# System initialization
si::sysinit:/etc/rc.d/rc.sysinit

In case of your distribution, your file may be different. I assume
that the file is /etc/rc.d/rc.sysinit (rc.sysinit for short)

�

Add the following piece of code to your initialization file
(rc.sysinit), in place of the "swapon -a" command. You may consider
not deleting but just commenting the original commands so that if
something goes wrong, we can restore the script to it's initial
state.

If your swapon is in /etc, replace /sbin/swapon with /etc/swapon.
If it is in /bin, replace with /bin/swapon. Do the same for
mkswap.

# -----------------------------------------------------------------------
# removed by yourname
# Start up swapping.
# action "Activating swap partitions" swapon -a
# -----------------------------------------------------------------------

# ----------------------------------------------------------------------
# added by yourname
# Verify and initialize swap space
#
/etc/rc.d/init.d/swapinit.sh
# ----------------------------------------------------------------------

�

A simple way to paste the text is to start two Virtual Consoles.
Log as a root in both cases. Use your favourite editor to open
this HOWTO on one console and the script to be modified on other
console. Now select the above code with your mouse and switch to
the other console. Place the cursor at the desired position to
paste and press the right mouse button. The selected text will be
copied.

�

Add the following piece of code to your shutdown file (halt).Put
this after any command that might need swap to be in place.
Normally, your halt file will have the "swapoff -a" command. First
replace the -a by /dev/winswap. Then immediately after that
section, add the following commands.

Note for RedHat Linux 6.1 : It seems that Red Hat Linux 6.1 halt
script automatically finds all the swap partitions and turns of
swapping. So, RedHat Linux 6.1 users need not replace the swapoff
$SWAPS comamnd by the swapoff -a command. In that case, just add
the /etc/rc.d/init.d/swaphalt.sh command to the halt file

# ------------------------------------------------------------------------
# Modified by yourname
# replaced "swapoff -a" by "swapoff /dev/winswap"
#
# Turn off swap, then unmount file systems.
runcmd "Turning off swap and accounting" swapoff /dev/winswap
[ -x /sbin/accton ] && /sbin/accton
# ------------------------------------------------------------------------

# ------------------------------------------------------------------------
# Added by yourname
# check swap signature and restore Windows swap info
/etc/rc.d/init.d/swaphalt.sh
# ------------------------------------------------------------------------

6.5. Reenable swapping

Uncomment the line in /etc/fstab that you commented earlier. (Not
really necessary, since we now do not refer to fstab for swap
partitions). Reboot Linux. You should now have swapping on the new
swap device.

7. A couple of notes

�

There is no need to add /dev/winswap to your /etc/fstab file. In
fact, it is probably wise not to do so (except possibly as a
comment).

�

If your Linux session crashes or otherwise exits without running
/etc/rc.d/init.d/halt, you will need to reboot and exit Linux
before swapping in Windows will work. It is also possible to
FORMAT X: and re-create the Windows swap file. The only way around
this would be to put the equivalent of the /.../halt commands in
the DOS AUTOEXEC.BAT file. I have tried to write some programs to
do this, but that turned out to be a hell lot of trouble ! First
and foremost, simply restoring the total special sectors doesn't
please DOS. Along with that, it seems you need to modify some
internal data structures so that DOS starts recognizing your drive,
otherwise DOS comes out with the error message:

Invalid media type reading drive X:
Abort, Retry or Fail ?

Since I am not aware of many details of DOS's or Windows's handling of
disk drives, I think at this point, you may consult some DOS expert.
Of course, if you are not concerned about the booting time, then you
can simply add the command to perform an unconditional FORMAT of X:
and then restore the TSS's. May be there are some other solutions. If
you are aware of one, please e-mail me today !

�

One way to restore your Windows swap partition is to start Linux,
disable swapping as described above, and then use the following
command :

# swapoff -a
# zcat /etc/winswap.gz > /dev/winswap
# reboot

�

Why to delete the RECYCLED folder ? According to the procedure
mentioned above, you must delete the RECYCLED folder. This is
necessary because we will be saving and restoring only the sectors
unto the root directory, but no further sectors. Also, we want the
drive X: to be restored to a "clean" state with no errors. If we
don't delete the RECYCLED folder, then the root directory entry is
restored, but the "." and ".." entries for this folder are not
restored and ScanDisk finds errors on the disk. To prevent this
from happening, we will delete the RECYCLED folder and hence only
the entry for win386.swp file will be restored.

�

If DOS' FDISK reports the partition as a "logical DOS drive", it
has a number of 5 (as in /dev/hda5) or higher. It is *NOT* the
partition labelled "extended" which has a number of 4 or less!! If
your Linux fdisk does not display logical partitions, you have a
broken Linux fdisk (Slackware 2.2 included a broken fdisk, for
example.) You can try "cfdisk" if your distribution has it, or you
will have to get a working fdisk.

�

When you turn off swapping, Linux has to manage everything in the
main memory itself. If your system memory is low, say something
like 16MB, then some large applications that run during
initialization may not be able to run properly. As an example, you
may get the following message during initialization:

Finding module dependencies Out of memory

I don't think this will cause any problems later until you reenable
swapping. But it's always better to take some precautions. As you will
be seeing in the next section, Linux can also be configured to use a
swap file instead of a swap partition. So if you feel that Linux will
not run properly with your normal system memory, create a swap file
that Linux will use temporarily until swapping is restored. This swap
file should preferably in the root directory. Figure out the minimum
size of swap space required. It will probably equal to your system
memory. Create a swap file of that size by giving the following
commands (assuming 16MB swap space):

# dd if=/dev/zero of=/swap bs=1024 count=16384
# mkswap /swap 16384
# sync

Now place the command swapon /swap just after the original swapon -a
command in the rc.sysinit file. Thus, now your system has been
configured to use swap file and hopefully there will be no Out of
memory errors. After having a workable swap partition (i.e after
following through this HOWTO completely), you can remove the swapon
/swap line that you added to your rc.sysinit file.

�

Please do not mail me unless you have checked and re-checked that
you copied your scripts correctly. More than half of the problems
reported with this Mini-HOWTO have been due to typos when copying
the scripts. (Use cut-and- paste if you can!). Even a spacing
error can cause an error. For example, if at the startup you get
the messages

Verifying swap space...Linux signature found
swapon: /dev/winswap invalid argument

Then, surely there has been a typing mistake in your scripts. Please
check your scripts.

�

Sometimes, the swap partition is detected to have neither Linux nor
Windows info and the "No signature found" error appears. I exactly
don't know why this happens, but for that case, I have written the
code in swap????.sh files to try six times before reporting an
error. Normally, the DOS signature should be found on the first
try, but many a times on my system it is found on iteration 2, 5 or
not at all after 12 iterations. You may consider increasing the
number of tries to some value like 12 or 24, there is no harm in
doing so. Once on my system, the script could not detect the
signatures even after 50 iterations ! If such a situation arises,
you may consider making a symbolic link to the swapinit.sh file in
your root home directory, and execute it at startup.

# ln -s /etc/rc.d/init.d/swapinit.sh swapinit.sh
# ./swapinit.sh

However, what I have observed is that once the setup runs on your
system for some time, the appropriate signatures are always found on
the first iteration.

�

For added reliability

Your Linux Box runs something very important, or your system memory
is less so that without swapping, your applications just start
collapsing ? You have set up the swap partition to be shared by
Windows. But many a times, your system is unable to detect the
proper signatures after exhaustive probing and so disables any
swapping partitions. At your own risk, you may consider removing
the signature checking logic, but here is a more approachable
solution.

First refer to the ``previous point'' and create a swap file in the
Linux root directory, with the name /swap. Keep it's size according
to your requirement. Now open the swapinit.sh file and modify the
last if statement as follows:

if [ $activated -ne 1 ] ; then
echo "Swap signature not found after $loopcount tries"
echo "Activating swap file"
swapon /swap
exit 1
fi

Thus if we find that swap partition lacks appropriate swap signatures,
then we enable the swap file. Since the swap file is on the root
partition and the Linux Kernel refuses to proceed until it mounts the
root file system, you are safely assured that the swap file will
always be successfully activated. Instead of creating and keeping the
swap file and wasting disk space, you may also modify the code to
create the swap file on the fly.

�

Saving more disk space As we saw, Linux and Windows can share their
swap spaces. But Window doesn't really require as large a swap file
as your Linux swap partition ? Then you can have some free space on
the swap partition (keep the Windows swap file size smaller than
the swap partition size). Swapping for Windows based applications
is managed by Windows itself. But many DOS programs like Turbo C++
Compiler, Norton DOS etc. also manage their own swap files. In that
case it is possible to configure these programs so that they create
their swap files in the swap partition instead of their default
location. This will increase your usable disk space. Also, most of
these program forget to delete the swap files they created, leading
to a large used disk space. But with this setup, every time you
shut down Linux, you get a clean swap partition, saving your time
in finding and deleting these swap files. For example, you can
tell Turbo C++ to swap on drive say E: by invoking it as

C:\CLAN>TC /RE

For Norton DOS, set the variable in the NDOS.INI file as,

Swapping = E:\

Many other programs may rely on the TEMP or TMP environment variables
to decide where they will swap. In that case you may set these vari�
ables appropriately in your AUTOEXEC.BAT file.

�

What if the swap partition is a FAT32 ? No, you can't have a swap
partition with a FAT32 file system. First of all, Linux can use a
maximum of 128MB of swap space on a single swap partition. Thus,
if your swap partition is larger than 128MB, you are wasting your
disk space. And Windows will refuse to convert a partition of 128MB
into a FAT32 file system.

�

The new style swap space Linux knows about two styles of swap
areas, the old style and the new style. By default, you will be
using the old style swap space with a signature "SWAP-SPACE". If
you want to use the new style swap space, then first of all change
"SWAP-SPACE" in all the above scripts to "SWAPSPACE2". Also add an
option -v1 in the mkswap command in the swapinit.sh script, as
follows

/sbin/mkswap -v1 /dev/winswap YYYYY

Also, run the following commands from the shell prompt :

# swapoff /dev/winswap
# mkswap -v1 /dev/winswap YYYYY
# swapon /dev/winswap

It seems that under new style swap space, your usable swap partition
can be as large as 2GB on i386 platforms. This means that you can now
have a swap partition larger than 512MB which Windows can format as a
FAT32 partition . Someone help on how do we save the "Total Special
Sectors" under a FAT32 partition.

�

For users of WINE WINE <http://www.winehq.com> is a program which
allows running Microsoft Windows programs on Linux. If you are
using WINE on a system that has been set up to share swap
partition, then when running a Windows program a win386.swp file of
about 8kb is created in your Windows folder. This is because the
swap file on X: is not available at that time. You can safely
delete this file once you finish running your Windows programs.

8. What are we exactly doing ?

�

Both Windows and Linux use swap space. When an OS is shut down, the
contents of the swap space are just garbage, the OS doesn't bother
about what is in it. Now what is the point in devoting a 60MB
partition for swap space to Linux and about 32MB of disk space
separately to Windows when only one of them is going to run at a
time (It is possible to run both Linux and Windows at the same
time, thanks to VMWare. For more information, see VMWare website
<http://www.vmware.com>). So we want Linux and Windows to share the
same hard disk space as swap space.

�

The problem is that Linux uses a partition as a swap space and
Windows uses a file as a swap space. ( Although it is possible to
create a swap file for Linux, it is not recommended. See "Linux
Installation and Getting Started" by Matt Welsh). Now if Linux uses
the same partition on which Windows stores it's swap file, it will
overwrite the boot sector, FAT and other data tables that Windows
assumes to exist on every drive. Thus, Windows will not be in a
position to find the swap file on startup and will create the
win386.swp file in the windows directory. Thus, even if by chance,
your Linux crashes, Windows will be in a position to start. In such
a case ( with Windows swap file in the Windows directory), just
restore the DOS/Windows swap info by restarting Linux and the start
Windows. Now Windows will be using the swap file on drive X: so you
can safely delete the win386.swp file in the windows directory.

�

The remedy is to store those critical data tables once on the Linux
partition in some file. Then every time Linux starts, check whether
the swap space was last used by Linux or Windows (using the label
you gave to your partition). If it was Linux, just enable swapping
else first make a swap partition on the X: drive using "mkswap
/dev/winswap" and then enable swapping.

�

When you shut down Linux, it is important to restore the swap
partition to the DOS format, so that Windows can start properly.
For that purpose we add the

/bin/zcat /etc/winswap.gz > /dev/winswap

command to the halt file.

9. The swapinit.sh script

This shell script initializes the swap space on the partition. The
code for signature detection has been repeated 6 times because many a
times the signature is properly detected only on the second or the
third try.

(Adapted from the original Swap-Space-HOWTO by H. Peter Anvin)

#!/bin/sh
#
# swapinit.sh - activate the swap partition
#
# written by Rahul U. Joshi
# Verify and initialize swap space
#

echo -n 'Verifying swap space... '

loopcount=0

# flag to indicate whether the partition has been activated or not
activated=0

# check for signatures 6 times before giving up
while [ $loopcount -lt 6 ]
do

if [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=10 skip=4086`" = 'SWAP-SPACE' ]; then
echo "Linux signature found, iteration $loopcount"
echo "Activating swap partitions"
swapon /dev/winswap
activated=1
break

elif [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=11 skip=43`" = 'SWAP SPACE ' ]; then
echo "DOS signature found, iteration $loopcount"
echo "Making swap partition"
mkswap /dev/winswap YYYYY
echo "Activating swap partitions"
swapon /dev/winswap
activated=1
break

else
let loopcount=loopcount+1
fi

done

if [ $activated -ne 1 ] ; then
echo "Swap signature not found after $loopcount tries"
echo "No swapping partitions activated"
exit 1
fi

10. The swaphalt.sh script

This script first checks the Linux swap signature and then restores
the Windows file system on it.

(Adapted from the original Swap-Space-HOWTO by H. Peter Anvin)

#!/bin/sh
#
# swaphalt.sh This file is executed through the /etc/rc.d/init.d/halt
# script after swapping and accounting has been turned off.
#
# Author: Rahul U. Joshi
#

# check swap partition signature and restore Windows swap info

loopcount=0

# flag to indicate whether the swap info has been restored or not
restored=0

# check for swap signature 3 times before giving up
while [ $loopcount -lt 3 ]
do

if [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=10 skip=4086`" = 'SWAP-SPACE' ]; then
echo "Restoring DOS/Windows swap info , iteration $loopcount"
/bin/zcat /etc/winswap.gz > /dev/winswap
restored=1
break
else
loopcount=loopcount+1
fi

done

if [ $restored -ne 1 ] ; then
echo "Swap signature not found after $loopcount tries"
echo "Skipping restoring"
fi

11. The msinfo.sh script

This shell script analyses the boot sector of the given partition and
displays some information along with the "Total Special Sectors" in a
message box. It assumes that the filesystem on the given partition is
a FAT16. If not, it will print an error message and exit. Invoke it as

# msinfo <partition name>

To run this script, you will need the "dialog" program that displays
dialog boxes. You can get it from here
<http://sunsite.unc.edu/pub/Linux/utils/shell>.

#!/bin/sh
#
# msinfo.sh This shell script displays the boot sector of the
# given partition.
#
# Author: Rahul U. Joshi
#
# Modifications Removed the use of expr and replaced it by the let
# command.
#
# ------------------------------------------------------------------------
# This program is a free software, you can redistribute it and/or modify
# it under the eterms of the GNU General Public Liscence as published by
# the Free Software Foundation; either version 2 or (at your option) any
# later version.
#
# This program is being distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY without even the implied warranty of
# MERCHANTIBILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
# Public Liscence for more details.
# -------------------------------------------------------------------------

# check for command line arguments
if [ $# -ne 1 ]; then
echo "Usage: msinfo <partition name>"
exit 1
fi

# check whether the input name is a block device
if [ ! -b $1 ]; then
echo "msinfo: $1 is not a block device"
exit 1
fi

# create two temporary files for use
TMPFILE=`mktemp -q /tmp/$0.XXXXXX`
if [ $? -ne 0 ]; then
echo "msinfo: Can't create temp file, exiting..."
exit 1
fi

TXTFILE=`mktemp -q /tmp/$0.XXXXXX`
if [ $? -ne 0 ]; then
echo "msinfo: Can't create temp file, exiting..."
rm -f $TMPFILE
exit 1
fi

backtitle="`printf "%78s" "msinfo, Information about FAT16 filesystem -- Rahul Joshi"`"

dialog --title "Boot sector of $1" --backtitle "$back_title" \
--infobox "\nAnalysing boot sector for $1\nPlease wait..." 14 60

# truncate TXTFILE to zero length
echo > $TXTFILE

# get Formatting DOS version
dd 2>/dev/null if=$1 bs=1 count=8 skip=3 | dd 2>/dev/null of=$TMPFILE
printf >>$TXTFILE "%30s : %s\n" "Formatting DOS version" "`cat $TMPFILE`"

# get file system
dd 2>/dev/null if=$1 bs=1 count=8 skip=54 | dd 2>/dev/null of=$TMPFILE
printf >>$TXTFILE "%30s : %s\n" "Filesystem" "`cat $TMPFILE`"

# check if filesystem in a FAT16
if [ "`cat $TMPFILE`" != "FAT16 " ]; then
dialog --title "Boot sector of $1" --backtitle "$back_title" \
--infobox "\nCan't find a FAT16 filesystem on $1" 14 60
exit 2
fi

# get volume label in boot sector
dd 2>/dev/null if=$1 bs=1 count=11 skip=43 | dd 2>/dev/null of=$TMPFILE
printf >>$TXTFILE "%30s : %s\n" "Volume label in boot sector" "`cat $TMPFILE`"

# get Sector size
dd 2>/dev/null if=$1 bs=1 count=2 skip=11| od -An -tdS | dd 2>/dev/null of=$TMPFILE
printf >>$TXTFILE "%30s : %d\n" "Sector size" `cat $TMPFILE`
sector_size=`cat $TMPFILE`

# get Reserved sectors
dd 2>/dev/null if=$1 bs=1 count=2 skip=14| od -An -tdS | dd 2>/dev/null of=$TMPFILE
printf >>$TXTFILE "%30s : %d\n" " Reserved sectors" `cat $TMPFILE`
reserved_sectors=`cat $TMPFILE`

# get FAT sectors
dd 2>/dev/null if=$1 bs=1 count=1 skip=16| od -An -tdS | dd 2>/dev/null of=$TMPFILE
fat_count=`cat $TMPFILE`

dd 2>/dev/null if=$1 bs=1 count=2 skip=22| od -An -tdS | dd 2>/dev/null of=$TMPFILE
sectors_per_fat=`cat $TMPFILE`

# calculate the no of sectors allocated for FAT's
let fat_sectors=fat_count*sectors_per_fat

printf >>$TXTFILE "%30s : %u (%u x %u) \n" "FAT sectors" "$fat_sectors" \
"$fat_count" "$sectors_per_fat"

# get root directory sectors
dd 2>/dev/null if=$1 bs=1 count=2 skip=17| od -An -tdS | dd 2>/dev/null of=$TMPFILE
root_sectors=`cat $TMPFILE`

# calculate the no of sectors allocated for root directory
let root_sectors=root_sectors*32/sector_size

printf >>$TXTFILE "%30s : %u\n" "Root directory sectors" "$root_sectors"

# get Total special sectors
let total=reserved_sectors+fat_sectors+root_sectors
printf >>$TXTFILE "%30s : %u\n" "Total special sectors" "$total"

# display the information
dialog --title "Boot sector of $1" --backtitle "$back_title" --msgbox "`cat $TXTFILE`" 14 60

# delete temporary files
rm -f $TMPFILE
rm -f $TXTFILE

# end of msinfo.sh

12. The original msinfo file

Here is the msinfo file required to find the "Total Special Sectors".
It was included by the original author of this HOWTO in an encoded
form. First copy the text from the line "begin 755 msinfo.gz" to the
line "end" into a temporary file say temp.uu. Then uudecode this file,
and finally gunzip to get msinfo file.

# uudecode temp.uu
# gunzip msinfo.gz

Now run the program using the command

# ./msinfo /dev/winswap

The program displays the boot sector information for given partition
as well as the "Total Special Sectors" for the drive.

begin 755 msinfo.gz
M'XL(`$$YNRT"`Y557TQ;51@_MW]HZ0AM'"$U-.28W([..>84%1%D2QH>',9*
M2S&VC);V=FV]T*;WWBD+1I)K'YH;DOJF#YH80GPR^J""D"R8-K('MS2;#Q*S
M%Y,E-P&592A$V:[?.;?M"FP/?DG/G^_[?7_Z_3G7PL313TT(?8P0<J"CI':8
M$5H^#:=5<KT^MQ`^UAQ1O*QUH0<.]B7S/FC[U!98Y;))]<*N^AG"^LB((J%E
MAFIM[A9U0O`;%:*7N#[L%G!_-IH34V(J,WUZ.CK%O6I#N4DTE,E-144Q-7T)
M>]_PX\M<3@!`'ZZ26[`-I7A.F!%$;JK.;21`!#.\-,5A/CK)\=B3FL:3F8PH
M<#$QDSO91Q%^>L%"Z@KW*"-NR3;""5SN,A?'NI[0=P0Q=#[P&&$5@3UNZ3VW
M=-(V`NYQ/)6CX)DC2F`KD!&C/!:R7"Q%]L-6`8'D+=]8T*^$V8@RR\;5\P9(
M^;\&T:F]E`Q`EI,_6YHCZBT#BJ09M0VV9!:XVKLF]0NX%'Z4R];\W](3A-%J
MI:`_&12!>@33QC0S'V8]?O5+0`;3UC23K)!"?@97N8R#:29MF/>R)PI>UJF)
MK%/-4QR39OSJE8,8MHZ)UC#`?1ZX'N!ZU`L'X;@.?X["C<1DUT%,9QW32C''
M"<98PQR#8#=(L'>96NQWR/4W1@>H3\'!OC3,MMN_"[,=A1OVK^XEH($[$S^H
MY@28=MB78-$/`=8)(L=\P*$5*I_LS@)_G7!=\Z9V@F"``Y$XQM[T!T>(WHNU
M/T9CH)YIZI("&9ZK32AR=DTN/QNZ."&7?27H_<(=_^=Q"U1N3Y.>!-E`$BY(
M;(4ZG)/[\_0F_54HY7\5[<!33)\2M%:Y(.THX_N%K?%2L9&(O=4>8F^@IV9(
M,<HEDU:17T'23M)GA#ANW-<F'J5-]>6!G2;BU+YN)@<RL.JBIFD4[:<###UG
MO3Z77WO?L?O+@@G\+(-<"X&QT"&#_N7@_\2;JGAEF&U1`JR#ZCD/Z]7HJ'ZH
M09_JNAZGJ^N?X5.39_AXMY!!?3@6G>X2,9^)QG%\!IZ@5`SSJ>EWN!SN>HCK
MLJ%F08R*J5B4YV=T0-R&J#UYJW>4#"7/#D,0/GG`0ZI@__`;6->]K!>C[<GD
M31/IQP>:!B]D?DT,*<;Y6=9Y]3:P][^W`?+>8F(1!*ZY7B1:BN=@D_XY9?0I
M'?ZT0=V@BBWIIV$47#Y@7-,9Q>0<*=7;NMW-NS2>9#_X3V)8()?A$;!`ALRW
M#&\%RO_^@9F6TP<]'M8J@'AFA"[JM]0(WCQ!`[.826!F&A@HG4JWU5R_KKL&
M9GNQF$;J[?NZ\YUZ?B].C,OEWI*\94J`:6LB?U,RK^2.([0QRQK(W!D2[K5M
M,G>&0B7A'K03F"'17*&L#9XU@+U2L9K4-GCIG(K(NE:SM,>SM,?;X,T:&U6,
MT./E%NAUTNE_$,^E(A0M!B7K[D'>X6'\FC25Q3W=+V3YLR^CXJW.S0>#54*#
M#ZE^AI+N-35^_>![U4!?@VQ%ET<L!BC>MC5"\GH->"T6'4/2NS>((N0,DT>_
9K6^!S%65KRS`,`,UVB43\!]-BKG]B`<``#5"
`
end

13. Acknowledgements, feedback and dedication

This Mini-HOWTO has been largely derived from the Swap-Space Mini
HOWTO by H. Peter Anvin. I have added the Windows 95/98 specific
details as well as a few Red Hat Linux specific details. The msinfo
program given in the original HOWTO didn't work on my Red Hat Linux 6
system, so I created a similar program. However, I have also included
the original msinfo program. Thanks to all those great people
involved with the development of Linux,to the people at Red Hat for
giving a great Linux distribution and to Sanjiv Guha, one of the co-
authors of "Red Hat Linux 6 Unleashed" from Techmedia for guidance
about shell programming. Thanks also to Matt Welsh, Greg Hankins and
Eric S. Raymond, the authors of SGML-Tool's Users Guide, which helped
me in converting the document in SGML format. Thanks are also to Peter
Norton and John Goodman for there venerable "Peter Norton's Inside the
PC", which helped me about DOS file system structure. Finally, thanks
to PC Quest Linux Project for bringing out Linux CDROMS frequently.

I rely on you, the readers, to make this HOWTO useful. If you have any
suggestions, comments, corrections, requests, flames, etc., feel free
to contact me. Right now, I have no internet access, but still you
contact me in case of questions at my Hotmail
<mailto:[email protected]> account (Hey! Don't blame me for using a
Microsoft service. Hotmail was created by our Indian guy Sabeer
Bhatia). May be someone from you local place may also come to your
rescue.If you find my account unoperational and your problem is indeed
important, you may mail me at [email protected]
<mailto:[email protected]>.

Finally, I would like to dedicate this work to Robert Lafore, the
author of excellent books "C programming using Turbo C++" and "Object
Oriented Programming using Turbo C++".

Enjoy Linux,

Rahul :-)