On Cluster Compute instances, its a bit more difficult. The cluster compute instances have their root filesystem in a partition inside of the disk attached. Thats all well and good, and most likely a partitioned disk is more familiar to you than one that is not partitioned. The cluster compute images have grub2 installed in the MBR of that disk.
The problem with the partition on the disk is that you can no longer simply launch the instance with a larger root volume and then 'resize2fs /dev/sda1'. This is because the kernel won't re-read the partition table of a disk until all of its partitions are unmounted. For the disk that holds your root partition, that means you basically have to reboot after a change.
To avoid that waste of precious time, we've included a utility called 'growpart' inside of the initramfs on the Ubuntu images. It is invoked by the 'cloud-initramfs-growroot' package. This code runs before the root fileystem is busy, so the request for the kernel to re-read the partition table will work without requiring a reboot. To try it out, do:
# us-east-1 ami-1cad5275 hvm/ubuntu-natty-11.04-amd64-server-20110426
$ ec2-run-instances --region us-east-1 --instance-type cc1.4xlarge \
--block-device-mapping /dev/sda1=:20 ami-1cad5275
When you get to the instance, you'll have a 20G filesystem on /. And, if you're interested enough to look in the console output, you'll see something like:
GROWROOT: CHANGED: partition=1 start=16065 old: size=16755795 end=16771860 new: size=41913585,end=41929650