Amazon VPC Reserved Instances vs EC2 Reserved Instances

We wanted to shine some light on AWS’ reserved instances and 

how they work with Virtual Private Clouds (VPC) and standard EC2.

What is VPC?
Amazon’s VPC or Virtual Private Cloud lets you provision a private isolated section of AWS’ cloud and launch resources as you require in a virtual network that you define. When defining your VPC, you have control over your network including IP address range, subnets and network gateways.

Why use AWS VPC?
Being able to define your own network when launching EC2 instances gives you a level of security control which is required in some use cases. For example, you may not require all of your EC2 instances to be internet accessible (e.g. your database) - with VPC, you can define which resources are internet facing and which ones are not. You can also define which resources are accessible via other nodes (other EC2 instances or even other VPCs). Read more about transitioning to VPC.

If you are not familiar with reserved instances and how they compare to on-demand instances, read our blog here: AWS Reserved Instances vs On-Demand: Breakeven point

EC2 Reserved Instances vs VPC Reserved Instances

When purchasing reserved instances, you have the option of selecting either standard EC2 instances or VPC EC2 instances under the platform drop down:

Purchasing VPC Reserved Instances

When do reserved instances apply:
If you have purchased standard EC2 reserved instances, those reservations will apply whether you launch a standard or VPC EC2 instance. However, if you have purchased VPC reservations, those reservations will apply ONLY to VPC EC2 instances, NOT to standard EC2 instances.

VCP Reserved Instances vs Standard Reserved Instances

From this, it might appear you should only ever buy standard EC2 reserved instances, however, there is a downside to doing this. The concept of reserved instances is not only to reduce your total bill, but to also guarantee your required level of capacity. Due to the fact that VPC hardware is separate to other publicly available resources, if you require a level of VPC resources and have not purchased VPC reserved instances, these resources are not guaranteed. This means that if you do not have VPC reserved instances and AWS’ VPC capacity is full, you will not be able to provision VPC resources.

VCP Reserved Instances vs Standard Reserved Instances

VPC Cost
There is no extra charge for using VPC other than your standard EC2 usage. However, if you choose to create a Virtual Private Network (VPN) connection to your VPC, you will be charged $0.05 per VPN connection-hour. Standard data transfer costs also apply on top of this.

Also note:

• CC1 instance types are not supported on VPC
• There is a limit of 5 VPCs per account per region
• There is a limit of up to 20 subnets per VPC

I hope that clears some of the questions around VPC and reserved instances. If you are looking to see how much reserved instances will save you, PlanForCloud is a free simulation engine you can use to get a detailed three year cloud cost forecast of your application on the public cloud.

Next blog post: Useful Cost Saving Tips On The Public Cloud

-- Hassan Hosseini
Product Manager at PlanForCloud

References
AWS Forum: https://forums.aws.amazon.com/message.jspa?messageID=372413
AWS Forum: https://forums.aws.amazon.com/thread.jspa?messageID=245831
AWS Site: http://aws.amazon.com/vpc/