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 have had an instance crash a few times on me, most notably when AWS had their 'little' EBS failure. Like you, I was unable to terminate the instances or detach the volume. I ended up creating a snapshot of the EBS volume (yes, it let me create a snapshot without detaching), creating a volume from that snapshot and attaching it as the root device on an instance.
Keep in mind that while the physical drive may not have been damaged, a crash can still damage the file system or the data.
I have also had success attaching the volume as an ordinary non-boot volume, running a file system check (e.g. e2fsck), and using rsync, in a procedure akin to what you would use to migrate from ephemeral/instance-store to EBS:
- Copy the root (/) directory to the EBS device (
rsync -aXHv)
- (optionally, rsync the devices as well (/dev), although I don't think it is needed)
- flush writes and unmount
The message I ended up 'taking home' was to have current backups even of EBS drives - so I now run ec2-consistent-snapshot frequently on data volumes and (less frequently) on my root volume, and rotate with ec2-prune-snapshots.
Hopefully some combination of the above (snapshot, check disk, rsync) can help you out.
(As an aside, the few other times I have seen this happen, I had some process running that consumed all the memory - and the particular AMI I was using didn't have any swap space setup - the console log (from the AWS console) is good for identifying that kind of problem)
Best Answer
By default, EC2 instance "terminate" automatically deletes all EBS volumes that were automatically created with the instance, but this can be changed. It does not,by default, delete EBS volumes that were attached after the instance started running, and this also can be changed.
Here's an article I wrote on how to protect your important data with EBS boot instances:
Note: Instance failure should not automatically delete your EBS volume. However, EBS volume failure is itself one failure mode, so make sure you are creating regular EBS snapshots. Not only does this give you a backup to rely on, but it also magically and transparently increases the reliability of the EBS volume itself.