Transform the /dev/xvd[b-z] devices on AWS to /dev/ephemeral[0-9]

make_ephemeral.sh

#!/bin/bash
#
# this script will attempt to detect any ephemeral drives and create /dev/ephemeral[0-9] device names
#
# Beware, This script is NOT fully idempotent.
#

METADATA_URL_BASE="http://169.254.169.254/2012-01-12"

# figure out the nameing convention
root_drive=`df -h / | grep -v grep | awk 'NR==2{print $1}' | sed -re 's/[a-z][0-9]$//' `

if [ "$root_drive" == "/dev/xvd" ]; then
  echo "Detected 'xvd' drive naming scheme (root: $root_drive)"
  DRIVE_SCHEME='xvd'
else
  echo "Detected 'sd' drive naming scheme (root: $root_drive)"
  DRIVE_SCHEME='sd'
fi

# figure out how many ephemerals we have by querying the metadata API, and then:
#  - convert the drive name returned from the API to the hosts DRIVE_SCHEME, if necessary
#  - verify a matching device is available in /dev/
#  - create the /dev/ephemeral[0-9] names 

ephemerals=$(curl --silent $METADATA_URL_BASE/meta-data/block-device-mapping/ | grep ephemeral)
for e in $ephemerals; do
  echo "Probing $e .."
  device_name=$(curl --silent $METADATA_URL_BASE/meta-data/block-device-mapping/$e)
  # might have to convert 'sdb' -> 'xvdb'
  device_name=$(echo $device_name | sed "s/sd/$DRIVE_SCHEME/")
  device_path="/dev/$device_name"

  # test that the device actually exists since you can request more ephemeral drives than are available
  # for an instance type and the meta-data API will happily tell you it exists when it really does not.
  if [ -b $device_path ]; then
    echo "Detected ephemeral disk: $device_path"
    mkfs.ext4 -F $device_path
    ln -s $device_path /dev/$e
  else
    echo "Ephemeral disk $e, $device_path is not present. skipping"
  fi
done