Server migration
Rehosting servers with Application Migration Service, one-off image imports with VM Import/Export, and the options for VMware estates.
Exam tasks: 4.2 (determine a new architecture for existing workloads when rehosting, select a migration tool for servers, plan cutover with minimal downtime)
The decision: do you need continuous replication with a cutover window of minutes, a one-time image import, or a way to keep running VMware without converting VMs at all?
Choosing a tool
| Application Migration Service | VM Import/Export | Amazon EVS | |
|---|---|---|---|
| Strategy | Rehost | Rehost, one server at a time | Relocate |
| Sources | Physical, any hypervisor, other clouds, other AWS Regions | VMDK, VHD/VHDX, OVA or raw images in S3 | VMware vSphere estates |
| Replication | Continuous, block level | None: point-in-time image | Uses VMware tools such as HCX |
| Downtime at cutover | Minutes | Hours: shut down, export, upload, import | Minutes with live migration tooling |
| Result | Native EC2 instances | AMI, EC2 instance or EBS snapshot | VMs on VMware Cloud Foundation in your VPC |
| At scale | Built for thousands of servers, grouped in applications and waves | Scripted per image | Whole clusters |
Name change
AWS Application Migration Service (MGN) now appears in AWS documentation as AWS Transform MGN. It's the same service and console, and the exam may use either name.
Application Migration Service (MGN)
Install the AWS Replication Agent on each source server, Windows or Linux. For VMware there's also an agentless option that replicates through a vCenter client, for servers where you can't install software.
Initial sync, then continuous replication. The agent copies every disk to replication servers in a staging subnet, then streams block-level changes. The staging area uses small instances and cheap EBS, so it costs little while you wait.
Configure launch settings. An EC2 launch template per server defines instance type, subnet, security groups and tags. MGN can right-size the instance type from the source's CPU and RAM.
Launch test instances. The conversion server makes the replicated disks bootable on EC2, with the right drivers and boot loader. Replication keeps running during tests, and you can test as often as you like.
Cut over. Stop the application on the source, wait for the last changes to replicate, launch cutover instances and switch DNS. Then finalize the cutover, which stops replication and cleans up the staging resources.
Post-launch actions run SSM documents on the new instances after launch, for test instances, cutover instances or both. Predefined actions cover installing the SSM Agent and CloudWatch agent, installing the Elastic Disaster Recovery agent for DR in another Region, license conversion and OS-level changes. You can add your own SSM documents, for example to join a domain or swap a hostname in config files.
Private replication path
If a question says replication must not use the public internet, configure the replication settings so the staging servers use private IP addresses, and route over Direct Connect or VPN. Add a VPC interface endpoint for MGN if the sources must reach the service endpoint privately too.
Forgetting the test launch
The cutover instance comes from the same replicated disks as the test instance. Answers that skip test launches, or that stop replication to test, miss the point: MGN lets you test repeatedly with no impact on the source or on replication.
MGN for DR
MGN is for migration. For ongoing disaster recovery with the same replication model, the answer is Elastic Disaster Recovery (DRS). See disaster recovery.
VM Import/Export
- Upload a disk image to S3, then call the import API to get an AMI, an instance or an EBS snapshot.
- Export turns an EC2 instance or AMI back into a VMDK, VHD or OVA image in S3, for on-premises or another cloud.
- Needs a service role, usually
vmimport, that lets the service read the S3 bucket. - No replication. The server is down, or diverging, from when you take the image until the import finishes.
Choose it for a handful of servers where downtime of hours is fine, for importing a golden image built on premises, or when you have to export an instance.
VMware estates
| Option | What it is | When it fits |
|---|---|---|
| Amazon Elastic VMware Service (EVS) | VMware Cloud Foundation deployed on EC2 bare metal, inside your own VPC. You bring VCF licenses | Relocate quickly while keeping vSphere, NSX and vSAN skills and tooling, with AWS-native networking |
| VMware Cloud on AWS | VMware-operated service on AWS infrastructure, now sold through Broadcom and partners | Existing VMC customers, or where VMware should operate the stack |
| AWS Transform for VMware | AI-assisted discovery, network conversion and wave planning that rehosts VMs onto EC2 through MGN | Leaving VMware entirely |
Relocate vs rehost for VMware
"No changes to the VMs", "keep using vCenter" or "exit the data center in weeks without converting images" points to relocate on EVS. "Reduce VMware licensing cost" or "run natively on EC2" points to rehost with MGN.
Legacy: use Application Migration Service instead
AWS Server Migration Service (SMS), which scheduled incremental VM snapshot replication through a connector, was discontinued in 2022. CloudEndure Migration was also discontinued, and its technology became MGN. CloudEndure Disaster Recovery became Elastic Disaster Recovery.
Scenarios
MGN replicates continuously from physical and any-hypervisor sources, allows unlimited test launches, and keeps cutover to minutes. VM Import/Export has no replication, so downtime would be hours and physical servers need imaging first. EVS needs VMware sources, and nightly backups can't meet a 30-minute cutover.
MGN replication can use private IPs over Direct Connect or VPN, and post-launch actions run SSM documents on the new instances, both predefined and custom. MGN doesn't replicate through an S3 bucket, VM Import/Export doesn't help with domain membership, and the replication port isn't configurable to 443.
Keeping vCenter, NSX and runbooks without converting VMs is the relocate strategy, and EVS runs VMware Cloud Foundation inside the customer's VPC. MGN converts VMs to EC2, which changes operations and needs re-testing. VM Import/Export is slow at this scale, and refactoring can't finish in 10 weeks.
Further reading
Data transfer
Choosing between DataSync, Transfer Family, Snowball Edge, S3 Transfer Acceleration, Direct Connect, VPN and Storage Gateway by data size, bandwidth, deadline and pattern.
Database migration
Moving databases to AWS with DMS full load and CDC, converting schemas for engine changes, using native tools, and keeping downtime short.