AWS EBS vs Instance Store

EC2 instances can be launched using either Elastic Block Store (EBS) or Instance Store volume as root volumes and additional volumes.

Instance Store (Ephemeral storage)

An Instance store backed instance is an EC2 instance uses Instance store as root device and accesses storage from disks that are physically attached to the host computer. Instance store provides temporary block-level storage for instances. The data on an instance store volume persists only during the life of the associated instance; if you stop or terminate an instance, any data on instance store volumes is lost.

Key points for Instance store backed Instance

1. Boot time is very fast usually less then a min, as the storage resides on the same machine
2. Can be selected as Root Volume and attached as additional volumes
3. Instance store backed Instances can be of maximum 10GiB volume size
4. Instance store volume can be attached as additional volumes only when is the Instance is launched and cannot be attached once the Instance is up and running
5. Instance store backed Instances cannot be stopped as one of the main reason being when stopped and started AWS does not guarantee the Instance would be launched in the same host.
6. Data on Instance store volume is LOST in following scenarios :-
• Failure of an underlying drive
• Stopping an EBS-backed instance where instance store are additional volumes
• Termination of the Instance
7. Data on Instance store volume is NOT LOST when the instance is rebooted
8. AMI creation requires usage on AMI tools and needs to be executed from within the server
9. Instance store backed Instances cannot be upgraded

Amazon Elastic Block Store (EBS)
An "EBS-backed" instance is an EC2 instance which uses an EBS volume as it’s root device.

An EBS volume behaves like a raw, unformatted, external block device that you can attach to a single instance and are not physically attached to the Instance host computer (more like a network attached storage). The volume persists independently from the running life of an instance. After an EBS volume is attached to an instance, you can use it like any other physical hard drive. You can also detach an EBS volume from one instance and attach it to another instance. EBS volumes can also be created as encrypted volumes using the Amazon EBS encryption feature.

Key points for EBS backed Instance

1. Boot time is slower then Instance store and usually less then 5 min
2. Can be selected as Root Volume and attached as additional volumes
3. EBS backed Instances can be of maximum 16TiB volume size depending upon the OS
4. EBS volume can be attached as additional volumes when the Instance is launched and even when the Instance is up and running
5. Data on the EBS volume is LOST only if the Root Volume is EBS backed and the Delete On Termination flag is checked (Checked by default)
6. Data on EBS volume is NOT LOST in following scenarios :-
• Reboot on the Instance
• Stopping an EBS-backed instance
• Termination of the Instance for the additional EBS volumes
7. EBS volumes are tied to a single AZ  in which they are created
8. EBS volumes are automatically replicated within that zone to prevent data loss due to failure of any single hardware component
9. AMI creation is easy using a Single command
10. EBS backed Instances can be upgraded for instance type, Kernel, RAM disk and user data

Exam Scenario Questions :-

1. EC2 EBS-backed (EBS root) instance is stopped, what happens to the data on any ephemeral store volumes?
• Data is automatically saved in an EBS volume.
• Data is unavailable until the instance is restarted.
• Data will be deleted and will no longer be accessible. 
• Data is automatically saved as an EBS snapshot.
2. When an EC2 instance that is backed by an S3-based AMI is terminated, what happens to the data on the root volume?
• Data is automatically saved as an EBS snapshot.
• Data is automatically saved as an EBS volume.
• Data is unavailable until the instance is restarted. 
• Data is automatically deleted.
3.  Which of the following will occur when an EC2 instance in a VPC (Virtual Private Cloud) with an associated Elastic IP is stopped and started? (Choose 2 answers)
• The Elastic IP will be dissociated from the instance
• All data on instance-store devices will be lost
• All data on EBS (Elastic Block Store) devices will be lost
• The ENI (Elastic Network Interface) is detached
• The underlying host for the instance is changed