EC2 Ephemeral Swap Helper The intent here is to initialize and active a swap partition from the local ephemeral disk, if any. Generally using an ephemeral disk is faster than EBS and you don't incur EBS IOPS overhead (read: cost) for swappy systems.

ec2-ephemeral-swapon.sh

#!/bin/bash
#
# Create a swap partition equal to the system RAM
# on the first ephemeral disk.
#

function activate_swap_on_device {
  swap_on=`swapon -s | grep 'dev' | wc -l`
  if [[ ${swap_on} -gt 0 ]]; then
    echo "At least one swap partition is already active. Doing nothing (we assume zero or one only)"
    return 3
  fi

  swap_list=`sfdisk -l $1`
  swap_check_status=$?

  swap_dev=`echo "${swap_list}" | grep -i "linux swap" | awk '{print $1}'`
  swap_count=`echo "${swap_dev}" | wc -l`

  # note - it'd be nice to check if the swap partition is already active

  if [[ ${swap_check_status} -eq 0 ]] && [[ ${swap_count} == 1 ]]
  then
    echo "Initializing swap on '${swap_dev}'"
    mkswap ${swap_dev}

    echo "Activating swap on '${swap_dev}'"
    swapon ${swap_dev}

    return 0
  elif [[ ${swap_check_status} -ne 0 ]]; then
    echo "Error checking for swap partitions on '$1'"
    return 2
  else
    echo "Found ${swap_count} swap paritions on '$1'. Expected one - doing nothing"
    return 1
  fi
}

device=`wget -q -O - http://169.254.169.254/latest/meta-data/block-device-mapping/ephemeral0`
device="/dev/${device}"

echo "Looking for block device '${device}'"

if [[ ! -b "${device}" ]]; then
  echo "Block device '${device}' not found. Looking for xvdX named device"
  device_id=`echo "${device}" | rev | cut -c 1`

  device_id_ord=`printf '%d' "'${device_id}"`
  device_id_ord=$(( ${device_id_ord} + 4 ))     # add four to map A->E, B->F, etc

  device_id_oct=`printf '%03o' ${device_id_ord}`
  device_id=`printf \\\\${device_id_oct}`

  device="/dev/xvd$device_id"
  echo "Looking for block device '${device}'"

  if [[ ! -b "${device}" ]]; then
    echo "Unable to find device. Bailing out..."
    exit
  fi
fi

echo "Found block device '${device}'"

sfdisk=`sfdisk -l ${device}`
sfdisk_empty_partitions=`echo "${sfdisk}" | grep -i "empty" | wc -l`

if [[ ${sfdisk} =~ "#blocks" ]] && [[ ${sfdisk_empty_partitions} -ne 4 ]]
then
  echo "Existing partitions found on '${device}'. Attempting to active existing swap (if any) "
  activate_swap_on_device ${device}
  echo "Done"
  exit
fi

swap_size=`grep MemTotal /proc/meminfo | awk '{print $2}'`

echo "Creating a swap parition of size ${swap_size}KB"
parted --align optimal --script -- ${device} mklabel msdos
parted --align optimal --script -- ${device} mkpart primary linux-swap 0 ${swap_size}KB

echo "Activating swap on '${device}'"
activate_swap_on_device ${device}

echo "Done"