Amazon recently announced an important new feature for their Elastic Compute Cloud. Previously, each instance was based on an image that could be a maximum of 10 GB in size. So, each machine you brought up could have a root partition up to 10 GB in size and additional storage would need to be added in other ways. The size restriction alone is somewhat limiting. Amazon has not only addressed that, but given users some other very powerful abilities.

Now, you can define an image in an EBS snapshot. That means the size of your root partition can be as large as 1 TB. Yes, that’s 100 times larger than the old 10 GB limit. Beyond the obvious benefit of having larger images, you can also stop instances. Stopping an instance is different than terminating an instance. The distinction is important because stopping an instance is very much like hitting the “pause” button. It doesn’t take a lot to realize that pausing a running instance and being able to start it up again later is very powerful! Instances tend to boot faster off EBS. As  you might expect, if you create a really large volume for a root partition (like 100s of GBs), it will take longer to come up. That’s just because it takes longer to create larger volumes than smaller ones.

Let’s go further and look at how powerful it is to have snapshots as the basis for images. By having a snapshot that you can create EBS volumes from, that means you can mount a volume, based on your snapshot (which represents your image) and make modifications to it! This is immensely helpful when trying to make changes to an image. Previously, it was somewhat more awkward to modify an image. You actually had to boot it up and run it. But now, even if there is an error that prevents proper running, you can access the image storage and make changes. Very useful!

Of course judging by the number of public AMIs out there, there are a great number of images backed by S3 that people will want to convert. Towards this end, I came up with a script to convert AMIs from the old to the new style. Here’s the cliff’s notes version.

Use an instance in the same region as your image to do the following,

• download the image bundle to the ephemeral store
• unbundle the image (resulting in a single file)
• create a temporary EBS volume in the same availability zone as the instance
• attach the volume to your instance
• copy the unbundled image onto the raw EBS volume
• mount the EBS volume
• edit /etc/fstab on the volume to remove the ephemeral store mount line
• unmount and detach the volume
• create a snapshot of the EBS volume
• register the snapshot as an image, and you’re done!

During the private beta for this feature, I created an AMI to handle all of this, so you boot the AMI with a set of parameters and it does the dirty work. The script uses the standard API and AMI tools that Amazon supplies. I’ll roll that out on the public cloud shortly.

Here’s the interesting portion of the script (parsing arguments and setting up environment variable for the tools has been omitted) :

AMI_DESC=`$EC2_HOME/bin/ec2dim |grep $AMI_ID`
MANIFEST=`echo $AMI_DESC | awk '{ print $3 }'`
ARCH=`echo $AMI_DESC | awk '{ print $7 }'`
MANIFEST_PATH=`dirname $MANIFEST`/
MANIFEST_PREFIX=`basename $MANIFEST |awk -F. '{ print $1 }'`

Download the bundle to /mnt

echo grabbing bundle $MANIFEST_PATH $MANIFEST_PREFIX
/usr/local/bin/ec2-download-bundle -b $MANIFEST_PATH -a $ACCESS_ID -s $SECRET_KEY -k pk.pem -p $MANIFEST_PREFIX -d /mnt

Unbundle the image into a single (rather large) file.

echo unbundling, this will take a while
/usr/local/bin/ec2-unbundle -k pk.pem -m /mnt/$MANIFEST_PREFIX.manifest.xml  -s /mnt -d /mnt

Create an EBS volume, 10 GB. This size is used because that is the largest size for an S3 based AMI. Using launch options I show at the end of this article, you can increase that at run time. Notice, the availability zone comes from instance metadata. We must wait till the volume is created before moving on.

ZONE=`curl http://169.254.169.254/latest/meta-data/placement/availability-zone`
VOL_ID=`$EC2_HOME/bin/ec2addvol -s 50 -z $ZONE | awk '{ print $2 }'`
STATUS=creating
while [ $STATUS != "available" ]
do
echo volume $STATUS, waiting for volume create...
sleep 3
STATUS=`$EC2_HOME/bin/ec2dvol $VOL_ID | awk '{ print $5 }'`
done

Attach the volume

INST_ID=`curl http://169.254.169.254/latest/meta-data/instance-id`
$EC2_HOME/bin/ec2attvol $VOL_ID -i $INST_ID -d $EBS_DEV

Here’s where we turn the image into a real volume, using our old friend “dd”

echo copying image to volume, this will also take a while
dd if=/mnt/$MANIFEST_PREFIX of=$EBS_DEV

Mount the volume and remove ephemeral store entry from /etc/fstab. This is required because “Boot from EBS” doesn’t use the ephemeral store by default.

mount $EBS_DEV /perm
cat /perm/etc/fstab |grep -v mnt >/tmp/fstab
mv /perm/etc/fstab /perm/etc/fstab.bak
mv /tmp/fstab /perm/etc/

Then, unmount and detach the volume. We’re nearly there.

umount /perm
$EC2_HOME/bin/ec2detvol $VOL_ID -i $INST_ID

Create a snapshot and wait for it to complete.

SNAP_ID=`$EC2_HOME/bin/ec2addsnap $VOL_ID -d "created by createAMI.sh" | awk '{ print $2 }'`
# now, wait for it
STATUS=pending
while [ $STATUS != "completed" ]
do
echo volume $STATUS, waiting for snap complete...
sleep 3
STATUS=`$EC2_HOME/bin/ec2dsnap $SNAP_ID | awk '{ print $4 }'`
done

Finally, delete the volume and register the snapshot

$EC2_HOME/bin/ec2delvol $VOL_ID
$EC2_HOME/bin/ec2reg -s $SNAP_ID -a $ARCH -d $DESCR -n $MANIFEST_PREFIX

To run your AMI with a larger root partition, use a command like this (which specifies 100GB);
  ec2-run-instances –key <KEYPAIR> –block-device-mapping /dev/sda1=:100 <AMI_ID>