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AWS recently introduced the ability to replace the root volume of EC2 instances using an updated AMI without stopping them. The Replace Root Volume helps patch the guest operating system and applications but still triggers a reboot of the instance.

The Replace Root Volume option allows developers to patch software quickly without having to perform instance store data backups or replication. Changing the AMI of a running instance will update applications and the operating system but will retain the instance store data, networking, and IAM configuration. An improvement on replacing root volumes using a snapshot, the new option can help developers with stateful workloads, simplifying the operating system’s patching and improving the deployment’s security.

Frank Fioretti, principal infrastructure architect at Huron Consulting Group, tweets:

This seems more like orchestration/automation than anything new really (…) For those using an Instance Store I can see the benefit in the event they want to swap out their root volume and maintain the instance store data.

One option of the new API is to restore a root volume to its launch state, with the replacement volume automatically restored from the snapshot that was used to create the initial volume during the launch. The replacement volume gets the same type, size, and delete on termination attributes as the original root volume. Jason Axley, principal security engineer at Amazon, tweets:

This makes using D.I.E. (Distributed Immutable Ephemeral) paradigm for cloud security way easier for legacy EC2: replace root volume by reverting to launch state.

According to the documentation, the EC2 remains on the same physical host, retaining its public and private IP addresses and DNS name. All network interfaces remain associated with the instance and all pending network traffic is flushed when the instance becomes available.

Corey Quinn, cloud economist at The Duckbill Group, comments in his newsletter:

Okay, this is awesome for a number of use cases. Sadly, it requires the instance to reboot quickly, but other than that it’s way more streamlined. Some people are going to hate this because it’s treating an instance as a pet instead of cattle, but… well, my development instance is a pet just as your laptop probably is to you.

A successful replacement task transitions through the following three states: pending, when the replacement volume is being created, in-progress, when the original volume is being detached and the replacement volume attached, and succeeded when the process completes and the instance is again available.

Replacing a root volume using an AMI will not change the encryption status of the root volume. If the AMI has multiple block device mappings, only the root volume of the AMI is used and the other volumes are ignored. If the instance supports the Nitro Trusted Platform Module (NitroTPM), the NitroTPM data for the instance is reset and new keys are generated.

The Replace Root Volume API is available in all AWS regions using the console, CLI, or SDKs. If performed using the AWS console, the new functionality is available in the new console only.