Mini-HOWTO: Linux/MS-Windows swap space sharing [revised]

Mini-HOWTO: Linux/MS-Windows swap space sharing [revised]

Post by H. Peter Anvin N9I » Sat, 16 Apr 1994 05:10:07

Dear Linux friends,
about two weeks ago I posted a mini-HOWTO about how to share swap
spaces between Linux and MS-Windows.  However, I have since gotten
feedback indicating some details were unique to my particular setup.
Hence, here is the revised version, that hopefully will work better.

Note: the included scripts have changed.  I am not 100% sure, but I
have a suspicion the problems stemmed from a different interpretation
of back quotes `..` in bash 1.12 and 1.13.  The revised versions
should work for all versions of sh.


                             A mini-HOWTO
                      Peter Anvin <>

                  Copyright (C) 1994 H. Peter Anvin

VERSION: 1.1                                            Date: 14 Apr 1994


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 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.


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.

        * MS-DOS 5.0 or newer [tested with 6.2]
        * MS-Windows 3.1 or newer
        * 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 in


* Boot DOS.  Create a DOS partition (using FDISK) the size = the size
  swap space you want.  It will be assigned a drive letter; 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.
        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.]
        LABEL X:
        DIR X:

* 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.

* 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 paritition 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.]

* The following is a shell script that analyzes the partition and
  derives some special information; extract it as "msinfo", set it
  executable and run it as:
        msinfo /dev/winswap

----[CUT HERE]----
# Extract special sector information from an MS-DOS partition


if [ "$#" != "1" ]; then
  echo "Usage: $0 <partition-name>"
  exit 1

if [ ! -r $1 ]; then
  echo "$1: Permission denied"
  exit 1

DOSVER="`dd 2>/dev/null if=$1 bs=1 count=8 skip=3`"
SECSIZE=`dd 2>/dev/null if=$1 bs=1 count=2 skip=11 | hexdump -e '1/2 "%u\n"'`
RESERV=`dd 2>/dev/null if=$1 bs=1 count=2 skip=14 | hexdump -e '1/2 "%u\n"'`
FATS=`dd 2>/dev/null if=$1 bs=1 count=1 skip=16 | hexdump -e '1/1 "%u\n"'`
ROOTDIR=`dd 2>/dev/null if=$1 bs=1 count=2 skip=17 | hexdump -e '1/2 "%u\n"'`
FATSIZE=`dd 2>/dev/null if=$1 bs=1 count=2 skip=22 | hexdump -e '1/2 "%u\n"'`
LABEL="`dd 2>/dev/null if=$1 bs=1 count=11 skip=43`"
FILESYS="`dd 2>/dev/null if=$1 bs=1 count=8 skip=54`"

if [ $EXTRA != 0 ]; then let ROOTSEC=$ROOTSEC+1; fi

echo "Formatting DOS version:      $DOSVER"
echo "Filesystem:                  $FILESYS"
echo "Volume label:                $LABEL"
echo "Sector size:                 $SECSIZE"
echo "Reserved sectors:            $RESERV"
echo "FAT sectors:                 $FATSEC (${FATS}x${FATSIZE})"
echo "Root directory sectors:      $ROOTSEC"
echo "Total special sectors:       $SPECIAL"
----[END OF SCRIPT]----

  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.  If it still does not work, please
  inform me about which version of DOS you are running, and I will try
  to help you out.

* [Optional step] Windows may occationally 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.

* 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

  For the remainder of this file, I will assume the filename was

* Type:

        dd if=/dev/winswap bs=512 count=XXX | gzip -9 > /etc/winswap.gz
  ... where XXX is replaced with the "Total special sectors" number.

* Add the following piece of code to your /etc/rc file (or whatever
  your init calls it), right before the command "swapon -a" (if there
  is no such command, add it to your /etc/rc file right before any
  mount commands).

  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.

  Replace XXXXX with the actual size of the partition in blocks, as
  given by fdisk.

# Verify and initialize swap space
echo -n 'Verifying swap space... '
if [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=10 skip=4086`" \
   = 'SWAP-SPACE' ]; then
  echo 'Linux signature found'
  /sbin/swapon /dev/winswap
elif [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=11 skip=43`" \
   = 'SWAP SPACE ' ]; then
  echo 'DOS signature found'
  /sbin/mkswap /dev/winswap XXXXX
  /sbin/swapon /dev/winswap
  echo 'No signature found'
  echo 'ERROR: Will not swap'

* Add the following piece of code to your /etc/brc file (or whatever
  it is called); put this after any command that might need swap to be
  in place.

# Terminate swapping and restore DOS/Windows swap info
/sbin/swapoff /dev/winswap
if [ "`/bin/dd 2>/dev/null if=/dev/winswap bs=1 count=10 skip=4086`" \
   = 'SWAP-SPACE' ]; then
  echo 'Restoring DOS/Windows swap info'
  /bin/zcat /etc/winswap.gz | /bin/dd of=/dev/winswap 2>/dev/null
  echo 'ERROR: /dev/winswap lacks swap signature, skipping restore'

* Reboot Linux.  You should now have swapping on the new swap device.


* There is no need to add /dev/winswap to your /etc/fstab file.

* If your Linux session crashes or otherwise exits without running
  /etc/brc, 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 swapfile.  The only way around this would be to put the
  equivalent of the /etc/brc commands in the DOS AUTOEXEC.BAT file;
  unfortunately I don't know of any way of doing that in DOS!

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