AWS can an EBS-backed instance also access "instance store?

Question

I thought I clearly understood the difference between instance-store and EBS backed AMIs.
But http://aws.amazon.com/maintenance-help/ says "if you are running an EBS-backed AMI, you can stop and then restart your instance in order to easily re-launch it. This will cause the loss of any data you have saved on the local instance store of the instance,"

Stop/start does NOT lose the sysvol data, so this confuses me.

I'm assuming that here, by "local instance store", they mean the backing EBS volume (the sysvol), and I'm thinking that they meant to say "terminate" instead of stop. Am I correct?

Answer 1

Terminating an EBS-backed instance will not cause your data to be deleted. You can still access the EBS volume until you delete it (unless you set it to delete when your instance is terminated).

Local instance store refers to hard drive space on the actual physical server that is running your instance. You can see the available instance store by doing sudo fdisk -l . Some images come with some instance store volumes already mounted (see df -h ). Otherwise you'll have to mount and format the instance store volumes before you can use them.

Data on an instance store volume is lost when you stop (not terminate) your instance because it is local to a physical server, and your instance might start up on a new server.

Answer 2

Quite simply, EC2 is running your virtual server on some physical server. The root filesystem can either be on a local disk (ephemeral storage) or on network attached storage (EBS). With EBS, they can snapshot it for backups or to make a copy, so EBS is far more flexible, although not as fast as a local disk in the server where your instance is running.

In order to make this all work, when you shutdown an ephemeral server, amazon wipes the disk in order to reallocate it to the next customer. There is no need or reason for them to do that with EBS, since it was not physically attached to that server in the first place.

You might note, that even EBS backed instances (depending on size) come with an allocation of ephemeral storage (2-500gig+) which can be used for swap, logs, or whatever else you want to do with them. The only issue of course is that should the server be shutdown, or should there be a catastrophic disk or hardware error, you'll lose that data. You can still manually back it up, in the same way people have backed up traditional servers over the years.

Making your own AMI from an EBS backed server is trivial now, and can be done easily through the AWS web interface. Making a non-EBS backed AMI is a very complicated task the last time I tried to do it. With that said, there are certain use cases where it makes a lot of sense to consider using purely ephemeral storage. Computation or memory/cache nodes that have no need to persist data will be faster and cost less.