How To Create a NFS Swap partition (OpenSlug version)

(see below for OpenDebianSlug notes)

First off, here's a shout-out to my friends (dyoung and robbat2) helping me out late at night (my timezone) and getting this job done. robbat2 is from the gentoo-embedded crew and explained this recipe to me. So no more delay... Here's the secret sauce.

Why do you want an NFS swap partition

I haven't run this on Unslung. It works on OpenSlug (I'll be testing some more on today's build 05/06/2005). While the slug is a great little workhorse, it's short on memory. Sometimes having a swap device comes in handy -- often while natively compiling. Sometimes one doesn't have a harddisk attached or any device attached, but a little extra memory would be great for performance or just to allow stuff to work.

How To do it

I'm assuming you are a little familiar with NFS and have an NFS server running.

1. Export a directory you want to have the swapfile in from the NFS server.
2. Change directory on the NFS server to that directory and create a swapfile. The following command creates a zeroed 128MB file called swapfile. As always be careful when using dd, and *double* check the command before you hit enter
   
   dd if=/dev/zero of=swapfile bs=1M count=128
3. Start the NFS server and portmap on the server machine if it's not already started.
4. On the TARGET (slug) NFS mount the partition. In my case it looked something like the following (192.168.1.5 is the NFS server IP address):
   
   mkdir /mnt/swap
mount -t nfs 192.168.1.5:/exported_NFS_directory /mnt/swap
5. On the TARGET, initialize the swapfile partition. N.B. I spent about 1 hour chasing down the fact that my Host and Target systems are opposite endian. So the Host created swapfile had a tiny little error when mounting the swap device. This is avoided by initializing the swapfile partition from the TARGET. The command to do this is
   
   mkswap /mnt/swap/swapfile
6. scp the loopback kernel module to the slug if you don't already have it. I'll be adding this as a default module pretty soon, but in the meantime you can just (from you build box find the loop.ko kernel module in build/tmp) and
   
   ipkg install kernel-module-loop
   
   Note: My build machine and my NFS server are the same device. Yours may be different.
7. From the slug target install the kernel module if one needs to:
   
   modprobe loop
8. loopmount the swap file as a loop device:
   
   losetup /dev/loop0 /mnt/swap/swapfile
9. Setup the device for swap usage:
   
   mkswap /dev/loop0
10. Mount the swap partition on the local loop device:
    
    swapon /dev/loop0
11. Enjoy!
12. cat /proc/meminfo to see the swap space is avialable to the system
    
    cat /proc/meminfo

Making it permanent

This is all good, but it can get a bit annoying doing that on every boot.

First, create the file /etc/init.d/swap (you may have to modify this a bit):

cat > /etc/init.d/swap
#!/bin/sh

set -e

case "$1" in
start)
echo -n "Starting swap: "
modprobe loop
losetup /dev/loop0 /swapfile
swapon /dev/loop0
echo "done"
;;
stop)
echo -n "Stopping swap: "
swapoff /dev/loop0
losetup -d /dev/loop0
echo "done"
;;
restart)
swapoff /dev/loop0
swapon /dev/loop0
;;
*)
echo "Usage: swap { start | stop | restart }" >&2
exit 1
;;
esac

exit 0
^D (that is pressing Ctrl+D)

Then make it executable:

chmod +x /etc/init.d/swap

Then make it start:

update-rc.d swap defaults

Done. On the next reboot it should mount everything.

OpenDebianSlug Notes

Getting the loopback module in OpenDebianSlug is a challenge (finding the equivalent of ipkg install kernel-loop-module). There may be easier ways, but one option is to do the following (different package for versions other than 2.7beta). Ideally you would do this in the initial install.

• reboot to flash (touch /.recovery and reboot)
• wget http://ipkg.nslu2-linux.org/feeds/openslug/cross/2.7-beta/kernel-module-loop_2.6.12.2-r17.1_nslu2.ipk
ipkg install kernel-module-loop_2.6.12.2-r17.1_nslu2.ipk
rm install kernel-module-loop_2.6.12.2-r17.1_nslu2.ipk
• mount your nfs filesystem on /mnt (mount host:/dir /mnt)
• cd /mnt
cp -Ra /lib/modules/* lib/modules
rm /mnt/.recovery
• reboot