The instance types described in the following table are available.

	Note
	The small instance type is the original Amazon EC2 instance type available since the launch of Amazon EC2. It is the default instance type for all customers. To use other instance types, you must specify them through the RunInstances operation.

	Important
	We strongly recommend using the 2.6.18 Xen stock kernel with the c1.medium and c1.xlarge instances. Although the default Amazon EC2 kernels work, the new kernels provide greater stability and performance for these instance types. For more information about kernels, see Kernels, RAM Disks, and Block Device Mappings.

Transitioning to a utility computing model changes how developers are trained to think about CPU resources. Instead of purchasing or leasing a particular processor to use for several months or years, you are renting capacity by the hour. Because Amazon EC2 is built on commodity hardware, over time there might be several different types of physical processors underlying different virtual EC2 instances. Our goal is to provide a consistent amount of CPU capacity regardless of the actual underlying hardware.

Amazon EC2 uses a variety of measures to provide each instance with a consistent and predictable amount of CPU capacity. To make it easy for developers to compare CPU capacity between different instance types, we defined an Amazon EC2 Compute Unit.

	Note
	We use several internal benchmarks and tests to manage the consistency and predictability of the performance of an Amazon EC2 Compute Unit. For more information, go to the Instance page.

To find out which instance works best for your application, we recommend launching an instance and using your own benchmark application. This helps you determine which instance type works best for your specific use case.

Amazon EC2 provides virtualized server instances. While some resources like CPU, memory and instance storage are dedicated to a particular instance, other resources like the network and the disk subsystem are shared amongst instances. If each instance on a physical host tries to use as much of one of these shared resources as possible, each receives an equal share of that resource. However, when a resource is under-utilized you are often able to consume a higher share of that resource while it is available.

The different instance types provide higher or lower minimum performance from the shared resources depending on their size. Each of the instance types has an I/O performance indicator (moderate or high). Instance types with high I/O performance have a larger allocation of shared resources. Allocating larger share of shared resources also reduces the variance of I/O performance. For most applications, moderate I/O performance is more than enough. However, for applications that require greater or more consistent I/O performance, consider instances with high I/O performance.