Not a problem, just a challenge!

We prefer to use UserData for automatic server setup. After some attempts it became clear that partitioning disks from a shell script is not exactly trivial tasks under Linux in AWS. BSD-based operating systems come with disklabel and fdisk and those will do the job. Linux comes with fdisk by default and that tool is somewhat limited …

Luckily, fdisk reads data from stdin so quick-and-dirty solution quickly emerged!

The following UserData is used to modify the instance store of a m3.large instance, creating 8GB swap partition and re-mounting the rest as /mnt:

#!/bin/bash -ex

# Mark execution start
echo "STARTING" > /root/user_data_run

# Unmount /dev/xvdb if already mounted
umount -f /dev/xvdb

# Partition the disk (8GB for SWAP / Rest for /mnt)
(echo n; echo p; echo 1; echo 2048; echo +8G; echo t; echo 82; echo n; echo p; echo 2; echo; echo; echo w) | fdisk /dev/xvdb

# Make and enable swap
mkswap /dev/xvdb1
swapon /dev/xvdb1

# Make /mnt partition and mount it
mkfs.ext4 /dev/xvdb2
mount /dev/xvdb2 /mnt
sed -i s/xvdb/xvdb2/g /etc/fstab

# Mark execution end
echo "DONE" > /root/user_data_run

Execute it with AWS CLI (Using stock Ubuntu 14.04 HVM AMI):

aws ec2 run-instances --image-id ami-1d8c9574 --count 1 --instance-type m3.large --key-name test-key --security-groups test-sg --user-data file://userdata.sh

The result:

:~> ssh ubuntu@ec2-54-197-66-121.compute-1.amazonaws.com "df -h"
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      7.8G  765M  6.6G  11% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            3.7G   12K  3.7G   1% /dev
tmpfs           749M  336K  748M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            3.7G     0  3.7G   0% /run/shm
none            100M     0  100M   0% /run/user
/dev/xvdb2       22G   44M   21G   1% /mnt
:~> ssh ubuntu@ec2-54-197-66-121.compute-1.amazonaws.com "free -h"
             total       used       free     shared    buffers     cached
Mem:          7.3G       276M       7.0G       352K       8.6M       177M
-/+ buffers/cache:        90M       7.2G
Swap:         8.0G         0B       8.0G
:~>

There it is, 8GB swap partition (/dev/xvdb1) and the rest (/dev/xvdb2) mounted as /mnt. Note that /etc/fstab is also updated to account for the device name change!