I am not entirely clear what the real value of "Marketplace" is, but we recently discovered a limitation that is quite precarious.
We've only tried a few of the Marketplace instances, but it seems that for any instance created from a Marketplace AMI, for example, your basic CentOS 6.4 minimal installation, you are unable to re-attach the root volume to another EC2 machine.
This is a pretty basic feature of anything that calls itself a "virtual disk".
We were sorely disappointed with this when we ran in to it. Thankfully, I wasn't in a bind. I wasn't in a situation where I was totally expecting to be able to do this because maybe I needed to salvage a system or its data.
After detaching the root volume from a Marketplace instance and attempting to re-attach it to another instance, you'll receive this error message:
"'vol-ffffffff' with Marketplace codes may not be attached as a secondary device."
|Screenshot of actual error message: "'vol-ffffffff' with Marketplace codes may not be attached as a secondary device."|
Here at Spry, we use EC2 instances from time to time. I've had to perform this operation many times in order to get a system back in working order or salvage the data. In each of those situations, it was paramount to our business that we be able to perform those function.
To us, something this major needs to be mentioned up front. If it weren't for a random user's review of a specific Marketplace offering, we would not have ever known! We would have gone ahead as normal. As if it were any other AMI. Then, it would come. The time when, we need to mount the EBS-backed storage to another machine. The EBS-backed storage that we've always been able to re-attach to another machine. The storage that we've been paying for. We would be out of luck.
IMO, this does not meet the definition of a virtual machine. With a virtual machine, there is this ability to use a virtual disk in all the ways that a virtual disk can and should be used. For instance, do things with it that you would expect to be able to do with a physical hard disk drive. Of course there are things like instance-storage which disappears when the machine is turned off. But before you even get too deep in to EC2, you would probably investigate the differences between EBS storage and the instance storage.