Amazon EC2 terminology - AMI vs. EBS vs. Snapshot vs. Volume

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.

I think let's make it simple. Create an AMI template from an existing instance (say instance#1. Note, when you create an AMI template, you will have a volume snapshot as well, look into your snapshot section. When you want to create new instance, choose the newly created AMI template, it will then pick the snapshot at the time the AMI template created. Simple.

Now, if you have been creating snapshots from the volume of instance#1, it is ok. Create new instance from the AMI template, then detach the volume that was automatically created for it, then attach volume created from snapshots from the volume of instance#1.

To summarize things:

• EBS = the AWS service itself

• EBS Volume = think of it like a hard drive you can attach to an EC2 instance

• Snapshot = a point in time copy of your volume

• AMI = a copy of a full instance

• EC2 = the service that RUNS the VM compute

Further to above explanations, here is an example to clarify all these.

Let's say your "EC2 Instance I1" has two EBS volumes attached to it - EBS Volume V1a and EBS Volume V1b.

Now, if you create an AMI image from EC2 Instance I1, you will get -

a. An AMI image of EC2 Instance I1, let's call it AMI1

b. A snapshot of EBS Volume V1a, let's call it S1

c. A snapshot of EBS Volume V1b, let's call it S2

Then, if you launch a new instance from AMI1 image, you will get -

a. A new EC2 instance, let’s call it I2

b. A new EBS Volume generated from Snapshot S1, let’s call it V2a

c. A new EBS Volume generated from Snapshot S2, let’s call it V2b

To sum it up -

1. An AMI image creates snapshot(s) of the volume(s) that are attached to the original instance (from which the AMI is created)

2. A new instance launched from an AMI image creates volume(s) from the snapshots attached to that AMI.

I explained it in detail in http://zilhaz.com/ebs-ami-aws-ec2/