Linux Swap Space Mini-HOWTO

• Index
• Introduction
• What you need
• Recent Changes and versions
• Administrative
• Before you begin
• The Procedure
• A couple of notes
• What are we exactly doing ?
• The swapinit.sh script
• The swaphalt.sh script
• The msinfo.sh script
• The original msinfo file
• Acknowledgements, feedback and dedication

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 variables 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 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.