Thursday 3 November 2022

AWS EC2 Instance Root Device


We can launch a new EC2 instance with e.g.:

$ aws ec2 run-instances \
--image-id ami-0179dbab9bb7ab8f7 \
--count 1 \
--instance-type t3.medium \
--key-name my-key-pair \
--security-group-ids sg-0eb70243855088ab1

Note here that we didn't need to explicitly specify Storage as default settings based on storage type set in AMI will be applied. This is the same as leaving default values in the Storage section when launching a new EC2 instance in AWS Console:

So, what is the storage (virtual hard disk...) where our EC2 instance boots from and where OS and applications are stored? It is called a root device.

From O’Reilly's AWS Certified Developer - Associate Guide by Vipul Tankariya, Bhavin Parmar:
Root device types
While choosing an AMI, it is essential to understand the root device type associated with the AMI. A bootable block device of the EC2 instance is called a root device. As EC2 instances are created from an AMI, it is very important to observe the root device type at the AMI. An AMI can have either of two root device types:
  • Amazon EBS-backed AMI (uses permanent block storage to store data)
  • Instance store-backed AMI (which uses ephemeral block storage to store data) 
While creating an EC2 instance using a web console, we can see whether an AMI is EBS- or instance-backed.

AWS EC2 Root Device Volume
When an EC2 instance is launched from an AMI, the root device volume contains the image used to boot the instance—mainly the operating system, all the configured services, and applications. This volume can be backed by either EBS or Instance Store (both are explained in the following sections) and can be configured when the AMI is created.
The Vector AMI is backed by EBS. This means that when you launch an EC2 instance from the Vector AMI, you will have at least one EBS volume attached to the instance, which will be the root volume. It will contain the operating system, a configured and ready-to-use Vector installation, sample data, and the sample database.

The root device is the virtual device that houses the partition where your filesystem is stored -- ephemeral d---evices have it running on the same physical host at the server, and EBS devices have it mounted using iSCSI.


Amazon EC2 instance root device volume - Amazon Elastic Compute Cloud

Amazon EBS volumes - Amazon Elastic Compute Cloud 

AWS EBS Ultimate Guide & 5 Bonus Features to Try 

AWS — Difference between EBS and Instance Store | by Ashish Patel | Awesome Cloud | Medium 

What Is EBS?. EBS is a popular cloud-based storage… | by Eddie Segal | Medium 

Create an Amazon EBS volume - Amazon Elastic Compute Cloud 

Tutorial: Get started with Amazon EC2 Linux instances - Amazon Elastic Compute Cloud 


It is important knowing what happens with EBS volumes when we stop and when we terminate EC2 instance.

From What is the difference between terminating and stopping an EC2 instance?

Amazon supports the ability to terminate or stop a running instance. The ability to stop a running instance is only supported by instances that were launched with an EBS-based AMI. There are distinct differences between stopping and terminating an instance.


Terminate Instance 

When you terminate an EC2 instance, the instance will be shutdown and the virtual machine that was provisioned for you will be permanently taken away and you will no longer be charged for instance usage. Any data that was stored locally on the instance will be lost. Any attached EBS volumes will be detached and deleted.


Stop Instance 

When you stop an EC2 instance, the instance will be shutdown and the virtual machine that was provisioned for you will be permanently taken away and you will no longer be charged for instance usage. The key difference between stopping and terminating an instance is that the attached bootable EBS volume will not be deleted. The data on your EBS volume will remain after stopping while all information on the local (ephemeral) hard drive will be lost as usual. The volume will continue to persist in its availability zone. Standard charges for EBS volumes will apply. Therefore, you should only stop an instance if you plan to start it again within a reasonable timeframe. Otherwise, you might want to terminate an instance instead of stopping it for cost saving purposes.

The ability to stop an instance is only supported on instances that were launched using an EBS-based AMI where the root device data is stored on an attached EBS volume as an EBS boot partition instead of being stored on the local instance itself. As a result, one of the key advantages of starting a stopped instance is that it should theoretically have a faster boot time. When you start a stopped instance the EBS volume is simply attached to the newly provisioned instance. Although, the AWS-id of the new virtual machine will be the same, it will have new IP Addresses, DNS Names, etc. You shouldn't think of starting a stopped instance as simply restarting the same virtual machine that you just stopped as it will most likely be a completely different virtual machine that will be provisioned to you.

If we try to terminate an instance via AWS Console we'll see a warning which states:
On an EBS-backed instance, the default action is for the root EBS volume to be deleted when the instance is terminated. Storage on any local drives will be lost.


No comments: