Planning and Executing Large-Scale Migrations with AWS MGN - Wave Design and Cutover Automation
Plan migrations of hundreds of servers using wave design, automate cutover with post-launch scripts, and optimize instances after migration.
Wave Design Principles
In large-scale migrations, you don't migrate all servers at once. Instead, you divide them into waves (migration batches) and execute them in stages. Wave design is based on application dependencies. Applications with tightly coupled web servers, application servers, and database servers should be placed in the same wave and cut over simultaneously. The first wave typically consists of low-risk applications (development environments, internal tools) to validate and refine the migration process. Subsequent waves handle production applications, and the final wave migrates the most critical systems. Use Migration Hub's application grouping feature to bundle servers by application and assign them to waves, preventing dependency oversights. A wave size of 10-25 servers is manageable and allows for effective troubleshooting during simultaneous cutover.
Cutover Automation
MGN's post-launch actions automatically execute scripts on EC2 instances after cutover. Integrated with Systems Manager Run Command, they can automate DNS record updates, monitoring agent installation, application configuration changes, and health checks. Application templates standardize the EC2 configuration for target instances (instance type, subnet, security group, IAM role, tags), applying consistent settings to servers with the same role. Since the same template is used for both test launches and cutover, configuration drift between test and production environments is eliminated. Execution order of post-launch actions is controlled by priority, enabling dependency-aware sequencing such as running DNS updates before monitoring agent installation. Pre-creating Systems Manager documents (SSM Documents) allows complex scripts to execute idempotently.
Post-Migration Optimization
Immediately after migration, run EC2 instances with the same specs as the on-premises servers and verify stability. After 2-4 weeks of operational data accumulates, right-size instance types based on AWS Compute Optimizer recommendations. Many on-premises servers were over-provisioned, and right-sizing can yield 30-50% cost savings. Consider migrating to Graviton (Arm-based) instances as well, which can deliver up to 40% cost reduction at equivalent performance. Post-migration optimization is not a one-time event. Establish a process to regularly review Compute Optimizer recommendations and continuously optimize costs. Apply Savings Plans or Reserved Instances during the post-migration stabilization period, avoiding long-term commitments while migration is still in progress. For a systematic approach to server migration from basics to advanced topics, books on Amazon offer comprehensive coverage.
MGN Pricing
MGN itself is free to use. Costs come from replication EC2 instances (lightweight t3.small instances created automatically), EBS volumes (for storing replication data), and EC2 charges for test and cutover instances. Since replication incurs ongoing costs, shortening the migration period is key to cost optimization. Terminate test instances promptly after validation, and stop replication for source servers after cutover. For data transfer costs, replication traffic from source servers to AWS (inbound) is free, but when source servers are on-premises, internet circuit or Direct Connect bandwidth costs apply.
Pre-Migration Assessment and Tool Selection
Before large-scale migration, use Migration Hub Discovery tools or Application Discovery Service to automatically collect server configurations, dependencies, and utilization from your on-premises environment. Feed this data into Migration Hub Strategy Recommendations to determine each server's migration strategy (Rehost / Replatform / Refactor). MGN is optimized for lift-and-shift (Rehost), replicating disks at block level without modifying the OS or applications. For database migration, use DMS (Database Migration Service) alongside MGN - the common pattern is MGN for application servers and DMS for databases. In VMware-heavy environments, MGN's agentless replication (vCenter connector) is also an option, eliminating the need to install agents on each server.
Risk Mitigation During Cutover
To minimize downtime during cutover, MGN's continuous replication performs block-level differential synchronization, with the final sync transferring only changed blocks. Actual cutover time per server is typically minutes to tens of minutes, depending on the volume of changed data rather than disk size. For rollback planning, keep source servers in standby rather than shutting them down, and pre-reduce DNS TTL (to 5 minutes or less) so you can instantly revert to original servers if issues arise. A two-stage process is recommended: perform test launches to validate all waves, approve test results, then proceed to cutover. When migrating to multiple regions, design wave placement order to ensure inter-region latency doesn't impact application-to-application communication.
Summary
The keys to successful large-scale migration are phased execution through wave design, cutover automation via post-launch actions, and post-migration optimization with Compute Optimizer. By combining MGN and Migration Hub, you can centrally manage migrations of hundreds of servers, minimizing risk while completing the migration efficiently. Combining pre-migration Discovery assessment with cutover rollback planning controls risk across the entire migration project.