An AMI, as you note, is a machine image. It's a total snapshot of a system stored as an image that can be launched as an instance. We'll get back to AMIs in a second.
Lets look at EBS. Your other two items are sub-items of this. EBS is a virtual block device. You can think of it as a hard drive, although it's really a bunch of software magic to link into another kind of storage device but make it look like a hard drive to an instance.
EBS is just the name for the whole service. Inside of EBS you have what are called volumes. These are the "unit" amazon is selling you. You create a volume and they allocate you X number of gigabytes and you use it like a hard drive that you can plug into any of your running computers (instances). Volumes can either be created blank or from a snapshot copy of previous volume, which brings us to the next topic.
Snapshots are ... well ... snapshots of volumes: an exact capture of what a volume looked like at a particular moment in time, including all its data. You could have a volume, attach it to your instance, fill it up with stuff, then snapshot it, but keep using it. The volume contents would keep changing as you used it as a file system but the snapshot would be frozen in time. You could create a new volume using this snapshot as a base. The new volume would look exactly like your first disk did when you took the snapshot. You could start using the new volume in place of the old one to roll-back your data, or maybe attach the same data set to a second machine. You can keep taking snapshots of volumes at any point in time. It's like a freeze-frame instance backup that can then easy be made into a new live disk (volume) whenever you need it.
So volumes can be based on new blank space or on a snapshot. Got that? Volumes can be attached and detached from any instances, but only connected to one instance at a time, just like the physical disk that they are a virtual abstraction of.
Now back to AMIs. These are tricky because there are two types. One creates an ephemeral instances where the root files system looks like a drive to the computer but actually sits in memory somewhere and vaporizes the minute it stops being used. The other kind is called an EBS backed instance. This means that when your instances loads up, it loads its root file system onto a new EBS volume, basically layering the EC2 virtual machine technology on top of their EBS technology. A regular EBS volume is something that sits next to EC2 and can be attached, but an EBS backed instance also IS a volume itself.
A regular AMI is just a big chunk of data that gets loaded up as a machine. An EBS backed AMI will get loaded up onto an EBS volume, so you can shut it down and it will start back up from where you left off just like a real disk would.
Now put it all together. If an instance is EBS backed, you can also snapshot it. Basically this does exactly what a regular snapshot would ... a freeze frame of the root disk of your computer at a moment in time. In practice, it does two things different. One is it shuts down your instance so that you get a copy of the disk as it would look to an OFF computer, not an ON one. This makes it easier to boot up :) So when you snapshot an instance, it shuts it down, takes the disk picture, then starts up again. Secondly, it saves that images as an AMI instead of as a regular disk snapshot. Basically it's a bootable snapshot of a volume.
Can i create a S3-backed /
instance-store AMI out of an EBS
based instance?
Yes, you can create an Instance-store AMI out of an EBS based instance.
- Install ec2-ami-tools in the instance
- Upload your x509 certificates to the instance
- Use ec2-bundle-vol and ec2-upload-bundle commands
NOTE: You need your account ID, Access Key and Secret Key to run these commands
- Register the image specifying the bucket/manifest name where you have uploaded the image files.
Step by step documentation can be found at http://docs.amazonwebservices.com/AWSEC2/2008-02-01/GettingStartedGuide/?ref=get-started
Refer the Creating an Image section of this guide.
Can i create S3-backed /
instance-store instance from EBS
based AMI?
No you can't
Best Answer
There are some advantages and disadvantages to EBS and instance store. In general, I recommend instance store over EBS for most scalable applications.
Some advantages of EBS:
Some advantages of instance store:
There are definitely more advantages and disadvantages than I have listed, but those are the ones that come to mind quickly. At work, we use instance store for all instances unless there's a very specific reason for having an EBS instance. All EBS instances are duplicated in every available availability zone within a region to help ensure some instances continue running in the event of another EBS outage.
I like to draw the line at customization: if an instance needs a lot of custom work to function, then use EBS, but if not, use instance store. I define custom work as tasks that need to be done by hand and are not automated. We use Puppet to deploy instances from scratch using the stock Ubuntu AMIs. I wrote a blog post about how we are able to take generic AMIs and place them in service without user intervention.
If you're going to use AutoScaling or similar technology, I highly recommend investing in automation, even if you will roll your own AMI, as there will likely be changes you'll need between AMI builds (code deployments, etc).
Regarding building instance store AMIs, there are a large number of tutorials available online showing how to build instance store AMIs, as instance store has been around a few years longer than EBS. The AMI tools are available at http://aws.amazon.com/developertools/368 .