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I'm getting a reserved AWS EC2 instance. but i'm worried that if i invest in it upfront and need to upgrade i wouldn't be able to.

In fact i would but i won't get my money back for the rest of the terminated instance.

Can i make multiple EC2 instances serve as one server, Have the same IP and Volume?

P.S. I am looking to install CentOS 6/WHM on it.

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Reserved instances are not tied to a particular EC2 instance. When you buy a reserved instance, it's basically allowing you to run any instance in a particular availability zone and size at a discount rate. But it can be any instance that meets the criteria at any given moment.

So if you buy a reserved instance, then upgrade that machine, the reserved instance won't apply anymore. However, if you started another instance (or had another instance running) of the same size at the same availability zone, then the reserved instance will start applying to that.

You cannot combine EC2 instances into a single machine.

EBS volumes can be attached only to a single EC2 instance at any given time.

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Some clarification here up front, as it seems you're not overly familiar with EC2 and how it all ties together.

An EC2 instance is a virtual machine running the operating system of your choice, and with a fixed CPU and RAM allocation. The amount of each is dictated by the instance type (t1.micro, m1.small, c1.medium etc.) See here for more information on instance types and specifications:

A reserved instance is a billing construct that is applied to your AWS bill. The way it works is that you pay a fixed sum up front based on a 1 or 3 year period, and this entitles you to a lower per-hour cost for your EC2 instance.

An EC2 instance type can be changed. You may stop the instance, change it (for example) from a t1.micro to a m1.large, and start it again, which upgrades the hardware available to the instance. If you wuld like a fixed IP address, you can associate an elastic IP address at no extra charge. Note that if you do NOT associate an elastic IP explicitly, your IP address will change on reboot and you will not be able to recover the address you were previously using.

If you purchase a reserved instance, to reduce the ongoing per-hour cost of running your EC2 instance and you later change the type of your instance, it is now possible to also adjust the type of your reserved instance. However, this is subject to some conditions, and AWS will not refund you any portion of the reserved instance price. See the following link for more information on how this works:

Merging multiple EC2 instances is not a feature of AWS or (as far as I know) any other existing cloud provider. If you want to spin up multiple instances and cluster them, you will need to know how to do so using Red Hat Cluster Suite, or the equivalent for your platform.

I hope this helps!

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According to this document is now possibile to merge or split reserved instances which belong to the same instance family.

Just make sure that your instances has been reserved at the same time (instance count): i.e. if you reserve simultaneously 10 m1.small instances today you will be able to group them into 5 m1.medium or into 1 m1.medium and 2 m1.large or into 1 m1.xlarge and 1 m1.medium, and so on.

BUT if you have reserved 5 m1.small yesterday, then you reserve 5 m1.small instances today, you will be able to only play at this game with two different groups: with the first group you can create 1 m1.large + 1 m1.small or 2 m1.medium + 1 m1.small; same story with the second group. In this case you will not be able to create a reservation for a m1.xlarge (because you don't have enough instances in the instance count).

PS: just to be clear, you can't merge instances: now you can merge the reservation and for that you can create (manually or creating an AMI starting from your available instances) a bigger/smaller instance which will be paid with the bigger/smaller reservation fees.

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