4.10 AMI's EBS Root Volumes vs. Instance Store

You can select your AMI based on:

• Region (see Regions and Availability Zones)

• Operating System

• Architecture (32-bit or 64-bit)

• Launch Permissions

• Storage for the Root Device (Root Device Volume)

  • Instance Store (Ephemeral Storage): You cannot stop your instance which is based on Instance Store, so you can only terminate it. When you launch an Instance Store based instance, you can attach other EBS volumes after your launching, but you can only attach other Instance Store volumes before your launching.

  • EBS Backed Volumes: When you go to Status Check, you can see the health report of your instance and the underlying hypervisor. If the hypervisor is in a degraded or failed state, what you can do is stopping the instance and then start it again and it will pre-provision on a new hypervisor so you can move the virtual machine around in the AWS data center by stopping and starting your instance. However, you cannot do this on a Instance Store. So if you have any instance which is Instances Store based, and it is on a failed host, you will lose this instance and its data, so Instance Store is called ephemeral storage, basically it means you get less durability than if you were to use EBS backed volumes.

  • You can detach your EBS based volume from an EC2 instance and attach it to another one even though it is a root device volume. However, you cannot do this for an volume of an instance which is Instance Store based, because it will not appear in your Volumes page of your console at all.

EBS vs. Instance Store - I

• All AMIs (actually, all block-storage device) are categorized as either backed by Amazon EBS or backed by Instance Store.

• For EBS volumes: The root device for an instance launched from the AMI is an Amazon EBS volume created from an Amazon EBS snapshot.

• For Instance Store volumes: The root device for an instance launched from the AMI is an instance store volume created from a template stored in Amazon S3.

EBS vs. Instance Store - II

• EBS backed instance:

  -->EBS volume--->(attach & mount)---->EC2 Instance<--
  |     |                                    |        |
  |     |                                    |        |
  --Snapshot--------->(register)----------->AMI--------

• Instance-store backed instance:

  Instance-store volume--->(attach & mount)---->EC2 Instance<--
     |                                               |        |
     |                                               |        |
  cannot create Snapshot                            AMI--------

• Difference between an EBS snapshot and an EBS boot AMI

  • An EBS boot AMI is an EBS snapshot of the EBS root volume plus some metadata like the architecture, kernel, AMI name, description, block device mappings, and more.

  • You can take a snapshot of an EBS boot volume and turn it into an EBS boot AMI by registering it with the appropriate metadata. The trickiest part of this is specifying the correct AKI id (kernel) so that it boots correctly.

Exam Tips

• Instance Store volumes are sometimes called Ephemeral Storage.

• Instance Store volumes cannot be stopped. It can only be terminated. If the underlying host fails, you have to terminate your instance, so you will lose your data.

• Instance store volumes cannot be taken snapshot or encrypted.

• EBS backed instances can be stopped. You will not lose the data on this instance if it is stopped.

• You can reboot both, you will not lose your data. Reboot will not change the underlying host machine, but stop or terminate will.

• By default, both Root volumes will be deleted on termination. However, with EBS volumes, you can tell AWS that you want to keep the root device volume through unchecking a checkbox when you launch or configure your instance.