AMIs

An Amazon EC2 instance can be launched from an AMI backed by Amazon S3 or an AMI backed by Amazon EBS. Instances launched from AMIs backed by Amazon S3 use an instance store as the root device (e.g., / or C:\). Instances launched from AMIs backed by Amazon EBS use Amazon EBS volumes as their root devices.

The following table describes the differences between AMIs backed by Amazon S3 and AMIs backed by Amazon EBS.

Characteristic	Instance Store	Amazon EBS
Boot Time	Usually Less than 5 minutes	Usually Less than 1 minute
Size Limit	10 GiB	1 TiB
Location	Instance storage	Amazon EBS volume
Data Persistence	Data persists for the life of the instance; non-root devices can use Amazon EBS	Data persists on instance failure and can persist on instance termination
Upgrading	Instance attributes are fixed for the life of an instance	The kernel, instance type, and ramdisk can be changed while the instance is stopped.
Charges	Instance usage and AMI storage	Instance usage, Amazon EBS volume usage, and AMI Storage
AMI Creation	Requires installation and use of AMI tools	Uses a single command/call
Stopped State	Cannot be in stopped state; instances are running or not	Can be placed in stopped state where instance is not running, but state is maintained in Amazon EBS

Public AMIs are available from Amazon and the Amazon EC2 community and can be downloaded from the Resource Center. You can use public AMIs as a base to create your own custom private AMIs.

Private AMIs are AMIs that you own and can only be accessed by you or those to whom you grant access.

Shared AMIs are AMIs that developers build and make available for other AWS developers to use. Building safe, secure, useable AMIs for public consumption is a fairly straightforward process, if you follow a few simple guidelines. For information on how to use shared AMIs and how to share AMIs, see Using Shared AMIs and How to Share AMIs.

Paid AMIs are AMIs that you purchase from developers or AMIs that come with service contracts from organization such as Red Hat.

Preparing and Creating an AMI

To use a file system image with Amazon EC2, you must prepare and create it as an AMI.

An Amazon EC2 instances can use its instance store (Amazon S3-backed) as its root device or an Amazon EBS volume (Amazon EBS-backed).

The creation process for an AMI that uses an instance store as its root device does the following:

Compresses the image to minimize bandwidth usage and storage requirements
Encrypts and signs the compressed image to ensure confidentiality and authenticates the image against its creator
Splits the encrypted image into manageable parts for upload
Creates a manifest file that contains a list of the image parts with their checksums

	Important
	After completion, you must register the AMI.

The creation process for an AMI that uses an Amazon EBS volume as its root device uses one of the following methods:

Creates snapshots for all attached Amazon EBS volumes and creates an AMI using a single command/call
Creates an AMI from a snapshot