How do I setup swap on Raspbmc?

Question

I have an external hard drive that has a 1GB swap partition. To use this, I log into my raspbmc via ssh and

sudo swapon /dev/sda2

Is there a way to automate this? I would like to check for the serial number of the disk and swapon if it is pluggedin. I would like to be able to do this each time it is pluggged in and not just at boot.

I would also like to swapoff when I attempt unmount the disk.

Where should I place my scripts? I am sure there is a script that fires when a disk is plugged in - the one that mounts the disk.

Answer 1

I'm the developer of Raspbmc, and I provided a simple method for installing swap (although it's disabled by default, and not recommended for 512MB Pis).

Simply run this via SSH:

touch /home/pi/.enable_swap

sudo reboot

You will now have a swap file created at /swap.

Source: http://svn.stmlabs.com/svn/raspbmc/release/oscore/upstart-wheezy/perf.conf

Raspbmc will install swap to USB if you are using a USB install (you checked install to USB in the Windows / Linux / OSX installers, or you put a file called USB on the vfat partition prior to installation)

Thanks

Sam Nazarko

Answer 2

Are you sure you really want to have swap on an external disk like that, over USB? It seems a little unusual, and might not be all that fast, so you might want to double check that requirement...

Assuming you do want to do that, the easiest way to have swap enabled at boot is via fstab. This would require the disk always be there, but if you can manage that, add an entry like:

UUID=b5958d5a-a89e-48c5-aea3-69296a43a50a none            swap    sw              0       0

To work out the UUID, one easy way is to cd to /dev/disk/by-uuid and do a ls -l. Look for the one that is symlinked to the partition you want (dmesg / logs should tell you the disk when you add it, fdisk -l can help you work out which partition on the disk you want)

Otherwise, if you want something that will enable swap when you add the disk, that will likely want to be a udev rule. This is quite a good tutorial. I suspect you'd want to write something that got all ACTION=="add", SUBSYSTEM=="usb" events and then in that script check the type and decide if it's your disk or not. If it is, have it mount your partitions, enable swap etc.

For shutting down the disk, I'd suggest a script that you run, which would both turn off swap and unmount the partitions. Anything else is likely to not fire at the right time, and Linux will object if the disk with swap on it vanishes... The script would be something like:

DISK_ID=wwn-0x500153203458ed3f
# TODO Find this more nicely!
DISK=`ls -l /dev/disk/by-id/$DISK_ID | awk '{print $11}' | awk -F'/' '{print $3}'`
if [[ "$DISK" == "" ]]; then
  echo "Disk not found!"
  exit 1;
fi

# Disable swap on partition
SP=`grep /proc/swaps /dev/$DISK | awk '{print $1}'`
if [[ "$SP" == "" ]]; then
   echo "No active swap found on $DISK"
   exit 1
fi
swapoff $SP

# Unmount other partitions on that disk
for PART in `mount | grep /dev/$DISK | awk '{print $3}'`; do
   echo "Unmounting $PART"
   umount $PART
done

# Mark disk as finished with
eject /dev/$DISK