During hardware failure, do EBS-based EC2 instances terminate or stop?

Amazon's new EBS-based EC2 instances have two options to shutdown: terminate or stop. Stopped instances can be later started again, automatically continuing from the same EBS root disk state they had when they were stopped.

But what happens when an Amazon datacenter has a hardware failure, and the EC2 instance is forced to shutdown. Does it terminate or stop? I开发者_如何学Pythonf the instance has been configured to stop by default on shutdown, can I rely on it being stopped also in this situation, and being able to start it again later?

An EC2 instance can be terminated at any time and one must account for this indeed, as already mentioned in David's answer (+1). You can arrange for a failed instance's Elastic Block Store (EBS) to remain available regardless though, see e.g. the respective FAQ What happens to my data when a system terminates?:

The data stored on a local instance store will persist only as long as that instance is alive. However, data that is stored on an Amazon EBS volume will persist independently of the life of the instance. If you are using an Amazon EBS volume as a root partition, then you have set the Delete On Terminate flag to "N" for your Amazon EBS volume to persist outside the life of the instance. [emphasis mine]

This is explained in more detail in section 2. Delete on Termination within Eric Hammond's recommended article Three Ways to Protect EC2 Instances from Accidental Termination and Loss of Data:

Though EBS volumes created and attached to an instance at instantiation are preserved through a “stop”/”start” cycle, by default they are destroyed and lost when an EC2 instance is terminated. This behavior can be changed with a delete-on-termination boolean value buried in the documentation for the --block-device-mapping option of ec2-run-instances.

He is referring to the ec2-run-instances documentation, and all this is meanwhile illustrated in more detail within Amazon EC2 Root Device Storage Concepts as well:

By default, the root device volume and the other volumes created when an Amazon EBS-backed instance is launched are automatically deleted when the instance terminates [...]. You can change the default behavior by setting the DeleteOnTermination flag to the value you want when you launch the instance. For an example of how to change the flag at launch time, see Using Amazon EC2 Root Device Storage.

I assume you mean the CPU related hardware fails, rather than the network disk. The way I treat EC2 is to create a system that can go up and down without data loss. Anything important you should use an S3 bucket, not EBS.