This can be done without creating a new AMI and without launching a new instance. When it s done the original root volume stays attached on /dev/sda1 (or wherever it was originally mounted. /dev/sda1 is the default for many AMIs). The original root volume will not be mounted to the filesystem - you d need to do that yourself via the "mount" command.
The technique requires the recent Ubuntu kernels, the ones that run in their 10.04 and 10.10 releases. Check out alestic.com for the most recent AMI IDs for these Ubuntu releases. These recent kernels are configured to boot from any attached device whose volume label is "uec-rootfs". If you are running one of these kernels all you need to do is to change the volume label of the current (instance-store) root volume to something else, change the volume label of the new root to uec-rootfs, and then reboot. If you re not running one of these kernels, you can t use this technique.
Here s the code. Put this in a file (reroot.sh) on the instance:
#! /bin/bash
device=$1
# change the filesystem labels
e2label /dev/sda1 old-uec-rootfs
e2label $device uec-rootfs
First you would attach the EBS volume you want to act as the new root to one of the available devices /dev/sdf../dev/sdp. This can be done either with direct EC2 API calls, with the EC2 Command Line API tools (ec2-attach-volume), or with a library such as boto, or via the AWS Management Console UI.
Then, run the reroot.sh script as root, and provide the device you attached the new root volume on, as follows:
sudo reroot.sh /dev/sdp
This will do the dirty work. Then you simply reboot:
sudo shutdown -r now
Viola.
To test this you should create an EBS volume that you know will boot properly. I like to do that by snapshotting the root volume of the EBS-backed AMIs from those above mentioned Ubuntu AMIs. From that snapshot you can create a new, bootable EBS volume in any Availability Zone. Make sure you can tell the difference between the running instance s original root volume and the new EBS root volume - before you run the reroot procedure above you can put in a "marker" file on the old root volume:
cd
touch this-is-the-original-root-volume
Then, when you reroot and reboot, if that file exists in your home directory you re still running with the original root volume. If it s not there, then the reroot-and-reboot worked.
Here are two example use cases for this technique, with thorough explanations:
http://shlomoswidler.com/2011/02/play-chicken-with-spot-instances/
http://shlomoswidler.com/2011/02/recapture-unused-ec2-minutes/