AWS Server Migration Service (SMS): The Legacy Service
AWS SMS was an agentless service designed to automate the migration of on-premises virtual machines, primarily from VMware vSphere and Microsoft Hyper-V environments, to the AWS cloud.
How SMS Worked
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SMS Connector: You would deploy a pre-configured virtual appliance called the SMS Connector in your on-premises vCenter environment. This connector communicated with AWS and managed the replication process without needing to install agents on individual VMs.
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Replication Jobs: In AWS, you would create and schedule replication jobs. SMS would then automate the process of taking snapshots of your live server volumes.
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Incremental Replication: SMS supported incremental replication, meaning it would only copy the changes since the last replication, which minimized network bandwidth usage. Each replication cycle created a new Amazon Machine Image (AMI).
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AMI Creation: The replicated server volumes were saved as new AMIs. These AMIs could then be launched as EC2 instances in your AWS account.
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Process: The underlying process involved temporarily replicating server volumes to an S3 bucket managed by the service, creating EBS snapshots from those volumes, and then creating the final AMI. The data in S3 was purged after the snapshots were created.
Key Characteristics of SMS
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Primary Use Case: Migrating large numbers of VMware or Hyper-V VMs to AWS.
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Agentless: Did not require software installation on the source servers.
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Output: The end product of a replication job was an AMI.
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Migration Type: Primarily supported a "Rehost" (Lift and Shift) migration strategy.
The Recommended Replacement: AWS Application Migration Service (MGN)
AWS Application Migration Service (MGN) is the modern, recommended service for migrating physical, virtual, and cloud-based servers to AWS. It is the successor to both SMS and CloudEndure Migration.
How MGN Works
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Agent Installation: You install a lightweight AWS Replication Agent on each source server.
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Continuous Replication: The agent performs continuous, block-level data replication from your source servers to a staging area in your AWS account. This keeps a real-time, up-to-date copy of your servers in AWS.
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Low-Cost Staging: The staging area consists of lightweight EC2 instances and low-cost EBS volumes, minimizing costs during the replication phase.
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Non-Disruptive Testing: You can launch test instances at any time without impacting the source servers, allowing you to validate the migration beforehand.
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Rapid Cutover: When you are ready to migrate, MGN automates the process of converting your replicated servers into fully provisioned EC2 instances, typically within minutes.
Why MGN is the New Standard
| Feature | AWS Server Migration Service (SMS) | AWS Application Migration Service (MGN) |
| :--- | :--- | :--- |
| Method | Agentless (via a connector appliance) | Agent-based (lightweight agent on each server) |
| Replication | Periodic, snapshot-based replication | Continuous, block-level replication |
| Cutover Window | Longer, dependent on last AMI creation | Very short, typically minutes |
| Source Platforms | VMware vSphere, Hyper-V | Physical servers, VMware, Hyper-V, other clouds |
| Testing | Limited to launching the last created AMI | Non-disruptive, on-demand test instance launches |
| Recovery Point | Point-in-time (when snapshot was taken) | Seconds behind the source |