LDP/LDP/howto/linuxdoc/Swap-Space.sgml

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<!-- Rahul Sundaram -->
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<article>

<title>Linux Swap Space Mini-HOWTO
<author>Rahul Sundaram <url url="mailto:rahulsundaram AT yahoo DOT co DOT in" name="rahulsundaram@yahoo.co.in">
<date>v1.6, 2002-12-27
<abstract>
This Linux Mini-HOWTO describes how to share your Linux swap partition with
Windows.
</abstract>
<toc>
<sect>Introduction
<p>
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.


<sect> What you need
<p>
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.

<p>
<enum>
<item>
   	MS-DOS 5.0 or newer and MS-Windows 3.1/3.11 <bf>OR</bf> MS-Windows 95/98
<item>
	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.)
</enum>
</p>

<sect> Recent Changes and versions
<p>
<itemize>
<item> Version 1.3 -- Written by H. Peter Anvin, and was not maintained
       from 15 Jan 1998.
<item> Version 1.4 -- Based on v1.3, but added more details.Rahul U. Joshi started maintainance
<item> Version 1.42 -- Deleted the recovery section for v1.4 users an v1.4
       never actually appeared on the LDP website.
<item> Version 1.5 -- Rahul U.Joshi stopped maintainance on 18 January 2000.Currently being maintained by Rahul Sundaram.
<item> Version 1.6 -- Renamed from Linux Swap space to this one.
</itemize>
</p>

<p>
Modifications by Rahul U. Joshi

Date: 26 Nov 1999
</p>

<p>
<itemize>
<item> Added Windows 95/98 specific details.
<item> Added Red Hat Linux specific details.
<item> Modified the msinfo program.
<item> Added sections 5 to 12.
<item> Completely revamped the "Procedure" section to make it more understandable.
<item> Split up the swap initialization and halt into two shell scripts.
<item> Modified the scripts to handle signature verification more than once.
<item> Added details about new type swap areas.
<item> Added more reliable schemes.
</itemize>
</p>


<sect>Administrative
<sect1>Copyright
<p>
This mini-HOWTO is Copyright  Rahul Sundaram.All rights reserved.This document is licensed under
the <url url="http://tldp.org/copyright.html" name="Linux Documentation Project license">.I welcome any kind of commercial distrubution but I would like to receive information regarding this.I would
also help anyone willing to translate this document.If you require any exceptions to the licensing terms
please contact me <url url="mailto:rahulsundaram AT yahoo DOT co DOT in" name="Rahul Sundaram">. The latest version  of this document is always available at the Linux Documentation
website at <url url="http://tldp.org/HOWTO/mini/Automount.html">.

<sect1>Disclaimer
<p>
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.


<sect>Before you begin
<p>
This is <bf>not</bf> 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.

<sect> The Procedure 
 
<p> 
<em><idx>NOTE:</idx></em>
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.  
</p> 
   
 
<sect1>  Turn off swapping and create a DOS partition 
<p>
<itemize>
<item>
<p>
Boot Linux. Turn off swapping by editing the /etc/fstab file and commenting 
the line that describes your swap partition. 

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

<item>
<p> 
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. 

<item>
<p>
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" 

<item>
<p>
Format this partition using the DOS FORMAT command. 
<tscreen> <verb>
C:\>FORMAT X: 
</verb></tscreen>

<item>
<p> 
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.] 
<tscreen><verb>
C:\>LABEL X:
C:\>DIR X:  
</verb></tscreen>

<item>
<p> 
At this stage, you have a DOS partition ready for use for swapping. 

  

</itemize> 
</p>
 
<sect1> Tell Windows the location of the new swap file 
   
<sect2><bf>For Windows 3.1 users</bf>
<p>
<itemize>
<item>
<p>
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. 


<item>
<p>
Exit Windows. 


</itemize>
</p>
 
<sect2><bf> For Windows 95/98 users </bf>
<p>
<itemize>
<item>
<p>   
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. 



<item>
<p> 
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. 

<item>
<p>
Now that you are not maintaining the Recycle Bin for drive X:, ideally the
<tt>RECYCLED</tt> 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:
<tscreen><verb>
C:\>X:
X:\>ATTRIB -R -H -S RECYCLED
X:\>DELTREE RECYCLED
</verb></tscreen>

Now you have drive X: containing only win386.swp. You can verify this
using the various options for the <tt>dir</tt>command. This does not mean
that the <tt>RECYCLED</tt> folder will not be there on drive X:. When
Windows boots up and finds that there is no <tt>RECYCLED</tt> 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 <tt>RECYCLED</tt> folder,
please e-mail me.


<item> 
<bf> Disk Cleanup troubles you ?  </bf>
<p>   
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.  


</itemize>
</p>
 
<p>  
>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.

</p> 
 
 
 
<sect1>Back up the Total Special Sectors 
<p>
<itemize>

<item>
<p> 
Boot Linux, then log in as root. 


<item>
<p> 
Use the fdisk command to find the name of the partition and its size in blocks.
  Create a symbolic link from <tt>/dev/winswap</tt> to this partition.  If the 
partition is hda7, then type: 
<tscreen><verb>
# ln -s /dev/hda7 /dev/winswap 
</verb></tscreen>
 
[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.] 



<item>
<p> 
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
<ref id="msinfo" name="msinfo.sh"> file using your favourite editor. Give the 
following commands at the  shell prompt 
<tscreen><verb>  
# cp Swap-Space-HOWTO msinfo.sh 
# chmod +x msinfo.sh 
</verb></tscreen>
 
(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 
<tscreen><verb> 
# ./msinfo.sh /dev/winswap 
</verb</tscreen>
   
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. 

<item>
<p> 
<bf>Note:</bf> 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 : 
<tscreen><verb>
Reserved Sectors at beginning : r
FAT Copies                    : f
Sectors per FAT               : s
Root directory entries        : r
</verb></tscreen> 

 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.


<item>
<p> 
[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: 
<tscreen><verb>
mkdir /mnt 
mount -t msdos /dev/winswap /mnt 
dd if=/dev/zero of=/mnt/dummy.fil 
umount /mnt   
</verb></tscreen>
 
The dd command will report "No space left on device".  This is exactly what 
you want. 


<item>
<p> 
Type on the shell prompt: 
<tscreen><verb> 
# dd if=/dev/winswap bs=512 count=XXX | gzip -9 > /etc/winswap.gz 
                                  ^^^
</verb></tscreen> 

                                    
... 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  


</itemize>
</p>
 
<sect1>   Modify the initialization and shutdown scripts to handle our new 
          configuration 
<p>
<itemize>

<item> 
<p> 
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. 



<item>
<p> 
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,
<tscreen><verb> 
# cp Swap-Space-HOWTO  /etc/rc.d/init.d/swapinit.sh
# cp Swap-Space-HOWTO  /etc/rc.d/init.d/swaphalt.sh
</verb></tscreen>

and then using your favourite editor, delete all execpt the required part. Now
 make these file "executable" by giving the following commands
<tscreen><verb>
# chmod +x swapinit.sh
# chmod +x swaphalt.sh
</verb></tscreen>


<item>
<p>
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. 



<item>
<p> 
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. 
<tscreen><verb>
# Runlevel 0 means shut down the system 
l0:0:wait:/etc/brc 
</verb></tscreen>
 
(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). 



<item> 
<p>
Check the name of your system initialization file. For SysVinit, this is the 
file listed in the following line of /etc/inittab. 
<tscreen><verb>     
# System initialization 
si::sysinit:/etc/rc.d/rc.sysinit 
</verb></tscreen>
     
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) 



<item> 
<p>   
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. 
<tscreen><verb> 
# ----------------------------------------------------------------------- 
# 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 
# ---------------------------------------------------------------------- 
</verb></tscreen>

 
<item>
 
<p> 
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. 



<item> 
<p> 
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. 

<p>
<em><idx>Note for RedHat Linux 6.1 : </idx></em> 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 <bf>not</bf> replace the <tt>swapoff
$SWAPS</tt> comamnd by the <tt>swapoff -a</tt> command. In that case, just
add the <tt>/etc/rc.d/init.d/swaphalt.sh</tt> command to the halt file

<tscreen><verb>  
# ------------------------------------------------------------------------ 
# 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 
# ------------------------------------------------------------------------ 
</verb></tscreen> 


</itemize>
</p>

 
<sect1> Reenable swapping 
<p> 
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. 

</p> 
 


<sect> A couple of notes 
<p> 
<itemize>
<item>
<p>
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). 



<item>
<p> 
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:
<tscreen><verb>
Invalid media type reading drive X:
Abort, Retry or Fail ?
</verb></tscreen>
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 !


<item>
<p> 
One way to restore your Windows swap partition is to start Linux, disable 
swapping as described above, and then use the following command : 
<tscreen><verb> 
# swapoff -a 
# zcat /etc/winswap.gz > /dev/winswap 
# reboot 
</verb></tscreen>


<item>
<p>
<bf>Why to delete the RECYCLED folder ?</bf>
According to the procedure mentioned above, you must delete the <tt>RECYCLED</tt>
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 <tt>RECYCLED</tt> 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 <tt>RECYCLED</tt> folder and hence only the entry for win386.swp file will be restored.



<item> 
<p> 
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. 


<item>
<label id="swapfile">
<p>
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:

<tscreen><verb>
Finding module dependencies Out of memory
</verb></tscreen>

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):

<tscreen><verb>
# dd if=/dev/zero of=/swap bs=1024 count=16384
# mkswap /swap 16384
# sync
</verb></tscreen>

Now place the command <tt>swapon /swap</tt> just after the original 
<tt>swapon -a</tt> 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 <tt>swapon 
/swap</tt> line that you added to your rc.sysinit file.



<item>
<p> 
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 
<tscreen><verb> 
Verifying swap space...Linux signature found 
swapon: /dev/winswap invalid argument 
</verb></tscreen>
 
Then, surely there has been a typing mistake in your scripts. Please check 
your scripts. 



<item>
<p> 
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. 
<tscreen><verb>       
# ln -s /etc/rc.d/init.d/swapinit.sh swapinit.sh 
# ./swapinit.sh  
</verb></tscreen>

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.
 
<item>
<p>
<bf>For added reliability</bf>

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. <bf>At your own risk</bf>, you may 
consider removing the signature checking logic, but here is a more
approachable solution. 

First refer to the <ref id="swapfile" name="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 <tt>if</tt> statement as follows:

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

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 <em>on the 
fly</em>.

 
<item>
<p>
<bf>Saving more disk space</bf>
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
<tscreen><verb>
C:\CLAN>TC /RE
</verb></tscreen>
For Norton DOS, set the variable in the NDOS.INI file as,
<tscreen><verb>
Swapping = E:\
</verb></tscreen>
Many other programs may rely on the <tt>TEMP</tt> or <tt>TMP</tt>
environment variables to decide where they will swap. In that case you may
set these variables appropriately in your AUTOEXEC.BAT file.


<item>
<p>
<bf> What if the swap partition is a FAT32 ? </bf>
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. 



<item>
<p>
<bf>The new style swap space </bf>
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 
<tscreen><verb> 
/sbin/mkswap -v1 /dev/winswap YYYYY 
</verb></tscreen>
   
Also, run the following commands from the shell prompt : 
<tscreen><verb>   
# swapoff /dev/winswap 
# mkswap -v1 /dev/winswap YYYYY 
# swapon /dev/winswap 
</verb></tscreen>
     
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. 

<item>
<p>
<bf>For users of WINE</bf>
<url url = "http://www.winehq.com" name="WINE"> 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 <tt>win386.swp</tt> 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.

</itemize> 
</p>
 
<sect> What are we exactly doing ? 
<p>
<itemize>

<item>
<p> 
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 <url url="http://www.vmware.com" 
name = "VMWare website">). So we want Linux and Windows to share the same hard
 disk space as swap space. 



<item> 
<p> 
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 <em><idx>"Linux Installation and Getting
Started"</idx></em> 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. 



<item>
<p> 
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. 


<item>
<p> 
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 
<tscreen><verb> 
/bin/zcat /etc/winswap.gz > /dev/winswap 
</verb></tscreen>
 
command to the halt file. 

</itemize> 
</p>
 
<sect> The swapinit.sh script 
<label id="swapinit">
<p>    
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) 
<tscreen><verb> 
#!/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 
</verb></tscreen>
</p> 
 
 
 
<sect> The swaphalt.sh script 
<label id="swaphalt">
<p>
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) 
 
<tscreen><verb> 
#!/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 
 
</verb></tscreen>
</p> 
 
 
<sect> The msinfo.sh script 
<label id="msinfo">
<p>    
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 
<tscreen><verb>
 
# msinfo <partition name> 
 
</verb></tscreen> 
 
To run this script, you will need the "dialog" program that displays dialog
boxes. You can get it from <url url="http://sunsite.unc.edu/pub/Linux/utils/shell"
name = "here">.

<tscreen><verb>
#!/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 
</verb></tscreen>
</p> 
 
 
<sect> The original msinfo file 
<p>    
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.  
<tscreen><verb> 
# uudecode temp.uu 
# gunzip  msinfo.gz  
</verb></tscreen>
      
Now run the program using the command 
<tscreen><verb>    
# ./msinfo /dev/winswap 
</verb></tscreen> 
The program displays the boot sector information for given partition as well 
as the "Total Special Sectors" for the drive. 
<tscreen><verb> 
begin 755 msinfo.gz 
M'XL(`$$YNRT"`Y557TQ;51@_MW]HZ0AM'"$U-.28W([..>84%1%D2QH>',9* 
M2S&ero;VC);V=FV]T*;WWBD+1I)K'YH;DOJF#YH80GPR^J""D"R8-K('MS2;#Q*S 
M%Y,E-P&592A$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&&$5@3UNZ3VW 
M=-(V`NYQ/)6CX)DC2F`KD!&ero;C/!:R7"Q%]L-6`8'D+=]8T*^$V8@RR\;5\P9( 
M^;\&ero;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&ero;(O=4>8F^@IV9( 
M,<HEDU:17T'23M)GA#ANW-<F'J5-]>6!G2;BU+YN)@<RL.JBIFD4[:<###UG 
MO3Z77WO?L?O+@@G\+(-<"X&ero;QT"&ero;#_N7@_\2;JGAEF&ero;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&ero;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&ero;A@HG4JWU5R_KKL& 
M9GNQF$;J[?NZ\YUZ?B].C,OEWI*\94J`:6LB?U,RK^2.([0QRQK(W!D2[K5M 
M,G>&0B7A'K03F"'17*&ero;L#9XU@+U2L9K4-GCIG(K(NE:SM,>SM,?;X,T:&ero;U6, 
MT./E%NAUTNE_$,^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 
</verb></tscreen> 
 
<sect>Acknowledgements, feedback and dedication
<p>
This Mini-HOWTO has been largely derived from the Swap-Space Mini HOWTO by
<bf>H. Peter Anvin</bf>. Rahul.U.Joshi 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 the Joshi's Red Hat Linux 6 system, so he created a similar  
program. However, He has also included the original msinfo program.It is now maintained by 
Rahul Sundaram.

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 <url url="mailto:rahulsundaram AT yahoo DOT co DOT in" name="Rahul Sundaram">.
I dedicate this document to my friends.

</p>

</article>