Amazon EC2 - Swap root instance store device with EBS device

Question

I have an EC2 instance with "instance store" device as a root device. Now, I would like to attach an EBS volume to that same instance, only that I want it to be the root device. Is that possible? What happens to the instance store device in such case?

Thanks in advance

Solution 1

You could migrate your running instance to an EBS backed AMI. Here's how I did it:

• Boot up a regular S3 AMI backed instance (or since you've already got an instance you're happy with, use that)
• Make an EBS volume of the same size as your root sda1 partition (currently the default is 10G for an m1.small and possibly others)
• Attach that EBS volume to a free block device on the instance using the web console or command line tools (e.g. /dev/sdd)
• Stop the services on the instance (e.g. /etc/init.d/mysql stop etc.)
• Copy the ephemeral root volume to the EBS volume:

dd bs=65536 if=/dev/sda1 of=/dev/sdd

• Check the EBS volume for consistency:

fsck /dev/sdd

• Mount the EBS volume onto the instance:

mount /dev/sdd /root/ebs-vol

• Remove the /mnt entry from the fstab on your EBS vol:

vim /root/ebs-vol/etc/fstab

• Unmout the EBS volume:

umount /dev/sdd

• Create a snapshot of the EBS volume using the AWS management console (or command line API tools)
• Make note of the snapshot id
• Register the snapshot image with AWS and take note of the AMI id produced, when registering remember to specify the kernel and ramdisk image (these should be the same as those used in your current instance):

ec2-register -s snap-12345 -a i386 -d "Description of AMI" -n "name-of-image" -k aki-12345 -r ari-12345

• To create an instance with more than 10G of persistent storage you have to use the cli tools. e.g. for 20G

ec2-run-instances ami-54321 -t m1.small -n 1 -g default --availability-zone=eu-west-1a -k ec2-key1 -b /dev/sda1=snap-12345:20:false

• If you boot up an instance based on one of these AMIs with > default volume size, once it's started up you can do an online resize of the filesystem:

resize2fs /dev/sda1

Solution 2

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/

Solution 3

You might also try the following tool to convert an instance-store AMI to an ebs-boot AMI: https://cloudyscripts.com/tool/show/2

Solution 4

AlexM has come up with good steps.

You would also be interested in checking this link: http://coderslike.us/2009/12/07/amazon-ec2-boot-from-ebs-and-ami-conversion/

EDIT: Another link: http://www.elastician.com/2009/12/creating-ebs-backed-ami-from-s3-backed.html

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Comments

[kdgregory]

+1 nice instructions, although I don't think it's what the OP wants.

[Sug]

Thank you very much for the comprehensive answer. That's great

[Michael Kohl]

We basically did the same, but used rsync with excludes instead of dd, worked like a charm too. One more thing to take care of (I know it's obvious, but it happens over and over again): when creating your EBS volume, make sure to double check it's in the same region as the instance you want to attach it to ;-)

[kdgregory]

+1, and thanks for making the note on my answer ... it's now deleted.

[John Hinnegan]

I didn't see an mnt entry here around the 4rth todo. vim /root/ebs-vol/etc/fstab

[Bill Horvath]

I tried following these instructions, but got hosed by dd: sh-4.1# dd bs=65536 if=/dev/sda1 of=/dev/sdf dd: writing `/dev/sdf': No space left on device 131073+0 records in 131072+0 records out 8589934592 bytes (8.6 GB) copied, 244.983 s, 35.1 MB/s sh-4.1# fsck /dev/sdf fsck from util-linux-ng 2.17.2 e2fsck 1.42.3 (14-May-2012) /: recovering journal Error writing block 2097153 (Invalid argument). Ignore error<y>? Suggestions?

[sourcedelica]

Shlomo - you rock! FYI - I had to rename my old EBS volume to cloudimg-rootfs from uec-rootfs, that appears to be the new boot label for Ubuntu AMIs as of today, but otherwise your advice saved my bacon.

[sourcedelica]

Although it's wierd, I tried to detach the original volume from /dev/sda1 but it wouldn't let me boot with my old volume on /dev/sdf. So I tried to reattach the old volume to /dev/sda1 but I was back to the problem where the BIOS says "Boot from hard disk failed. Not a bootable disk". The only thing that works is the original EBS on /dev/sda1 and the old EBS on /dev/sdf