About the Migration Toolkit for Containers

The Migration Toolkit for Containers (MTC) enables you to migrate stateful application workloads between OKD 4 clusters at the granularity of a namespace.

You can migrate applications within the same cluster by using state migration.

MTC provides a web console and an API, based on Kubernetes custom resources, to help you control the migration and minimize application downtime.

The MTC console is installed on the target cluster by default. You can configure the Migration Toolkit for Containers Operator to install the console on a remote cluster.

See Advanced migration options for information about the following topics:

• Automating your migration with migration hooks and the MTC API.

• Configuring your migration plan to exclude resources, support large-scale migrations, and enable automatic PV resizing for direct volume migration.

MTC terminology

1 Called the target cluster in the MTC web console.

MTC workflow

You can migrate Kubernetes resources, persistent volume data, and internal container images to OKD 4.6 by using the Migration Toolkit for Containers (MTC) web console or the Kubernetes API.

MTC migrates the following resources:

• A namespace specified in a migration plan.

• Namespace-scoped resources: When the MTC migrates a namespace, it migrates all the objects and resources associated with that namespace, such as services or pods. Additionally, if a resource that exists in the namespace but not at the cluster level depends on a resource that exists at the cluster level, the MTC migrates both resources.

  For example, a security context constraint (SCC) is a resource that exists at the cluster level and a service account (SA) is a resource that exists at the namespace level. If an SA exists in a namespace that the MTC migrates, the MTC automatically locates any SCCs that are linked to the SA and also migrates those SCCs. Similarly, the MTC migrates persistent volume claims that are linked to the persistent volumes of the namespace.

  Cluster-scoped resources might have to be migrated manually, depending on the resource.

• Custom resources (CRs) and custom resource definitions (CRDs): MTC automatically migrates CRs and CRDs at the namespace level.

Migrating an application with the MTC web console involves the following steps:

1. Install the Migration Toolkit for Containers Operator on all clusters.

   You can install the Migration Toolkit for Containers Operator in a restricted environment with limited or no internet access. The source and target clusters must have network access to each other and to a mirror registry.

2. Configure the replication repository, an intermediate object storage that MTC uses to migrate data.

   The source and target clusters must have network access to the replication repository during migration. If you are using a proxy server, you must configure it to allow network traffic between the replication repository and the clusters.

3. Add the source cluster to the MTC web console.

4. Add the replication repository to the MTC web console.

5. Create a migration plan, with one of the following data migration options:

   • Copy: MTC copies the data from the source cluster to the replication repository, and from the replication repository to the target cluster.

     If you are using direct image migration or direct volume migration, the images or volumes are copied directly from the source cluster to the target cluster.

     

   • Move: MTC unmounts a remote volume, for example, NFS, from the source cluster, creates a PV resource on the target cluster pointing to the remote volume, and then mounts the remote volume on the target cluster. Applications running on the target cluster use the same remote volume that the source cluster was using. The remote volume must be accessible to the source and target clusters.

     Although the replication repository does not appear in this diagram, it is required for migration.

     

6. Run the migration plan, with one of the following options:

   • Stage copies data to the target cluster without stopping the application.

     A stage migration can be run multiple times so that most of the data is copied to the target before migration. Running one or more stage migrations reduces the duration of the cutover migration.

   • Cutover stops the application on the source cluster and moves the resources to the target cluster.

     Optional: You can clear the Halt transactions on the source cluster during migration checkbox.

About data copy methods

The Migration Toolkit for Containers (MTC) supports the file system and snapshot data copy methods for migrating data from the source cluster to the target cluster. You can select a method that is suited for your environment and is supported by your storage provider.

File system copy method

MTC copies data files from the source cluster to the replication repository, and from there to the target cluster.

Snapshot copy method

MTC copies a snapshot of the source cluster data to the replication repository of a cloud provider. The data is restored on the target cluster.

AWS, Google Cloud Provider, and Microsoft Azure support the snapshot copy method.

Direct volume migration and direct image migration

You can use direct image migration and direct volume migration to migrate images and data directly from the source cluster to the target cluster.

Direct migration has significant performance benefits because it skips the intermediate steps of backing up files from the source cluster to the replication repository and restoring files from the replication repository to the target cluster.

Direct migration uses Rsync to transfer the files.

State migration

You can use Migration Toolkit for Containers (MTC) to migrate an application’s state.

State migration copies selected persistent volume claims (PVCs) and Kubernetes objects that constitute an application’s state.

If you have a CI/CD pipeline, you can migrate stateless components by deploying them on the target cluster. Then you can migrate the application’s state by using MTC.

You can migrate an application within the same cluster by using state migration.