Advanced migration options

You can automate your migrations and modify the MigPlan and MigrationController custom resources in order to perform large-scale migrations and to improve performance.

MTC terminology

1 Called the target cluster in the MTC web console.

Migrating applications by using the CLI

You can migrate applications with the MTC API by using the command line interface (CLI) in order to automate the migration.

Migration prerequisites

• You must be logged in as a user with cluster-admin privileges on all clusters.

Direct image migration

• You must ensure that the secure internal registry of the source cluster is exposed.

• You must create a route to the exposed registry.

Direct volume migration

• If your clusters use proxies, you must configure an Stunnel TCP proxy.

Clusters

• The source cluster must be upgraded to the latest MTC z-stream release.

• The MTC version must be the same on all clusters.

Network

• The clusters have unrestricted network access to each other and to the replication repository.

• If you copy the persistent volumes with move, the clusters must have unrestricted network access to the remote volumes.

• You must enable the following ports on an OKD 4 cluster:

  • 6443 (API server)

  • 443 (routes)

  • 53 (DNS)

• You must enable port 443 on the replication repository if you are using TLS.

Persistent volumes (PVs)

• The PVs must be valid.

• The PVs must be bound to persistent volume claims.

• If you use snapshots to copy the PVs, the following additional prerequisites apply:

  • The cloud provider must support snapshots.

  • The PVs must have the same cloud provider.

  • The PVs must be located in the same geographic region.

  • The PVs must have the same storage class.

Creating a registry route for direct image migration

For direct image migration, you must create a route to the exposed internal registry on all remote clusters.

Prerequisites

• The internal registry must be exposed to external traffic on all remote clusters.

  The OKD 4 registry is exposed by default.

Procedure

• To create a route to an OKD 4 registry, run the following command:

  $ oc create route passthrough --service=image-registry -n openshift-image-registry

Configuring proxies

For OKD 4.1 and earlier versions, you must configure proxies in the MigrationController custom resource (CR) manifest after you install the Migration Toolkit for Containers Operator because these versions do not support a cluster-wide proxy object.

For OKD 4.2 to 4.8, the Migration Toolkit for Containers (MTC) inherits the cluster-wide proxy settings. You can change the proxy parameters if you want to override the cluster-wide proxy settings.

You must configure the proxies to allow the SPDY protocol and to forward the Upgrade HTTP header to the API server. Otherwise, an Upgrade request required error is displayed. The MigrationController CR uses SPDY to run commands within remote pods. The Upgrade HTTP header is required in order to open a websocket connection with the API server.

Direct volume migration

If you are performing a direct volume migration (DVM) from a source cluster behind a proxy, you must configure an Stunnel proxy. Stunnel creates a transparent tunnel between the source and target clusters for the TCP connection without changing the certificates.

DVM supports only one proxy. The source cluster cannot access the route of the target cluster if the target cluster is also behind a proxy.

Prerequisites

• You must be logged in as a user with cluster-admin privileges on all clusters.

Procedure

1. Get the MigrationController CR manifest:

   $ oc get migrationcontroller <migration_controller> -n openshift-migration

2. Update the proxy parameters:

   apiVersion: migration.openshift.io/v1alpha1
   kind: MigrationController
   metadata:
     name: <migration_controller>
     namespace: openshift-migration
   ...
   spec:
     stunnel_tcp_proxy: http://<username>:<password>@<ip>:<port> (1)
     httpProxy: http://<username>:<password>@<ip>:<port> (2)
     httpsProxy: http://<username>:<password>@<ip>:<port> (3)
     noProxy: example.com (4)

   1	Stunnel proxy URL for direct volume migration.
   2	Proxy URL for creating HTTP connections outside the cluster. The URL scheme must be http.
   3	Proxy URL for creating HTTPS connections outside the cluster. If this is not specified, then httpProxy is used for both HTTP and HTTPS connections.
   4	Comma-separated list of destination domain names, domains, IP addresses, or other network CIDRs to exclude proxying.

   Preface a domain with . to match subdomains only. For example, .y.com matches x.y.com, but not y.com. Use * to bypass proxy for all destinations. If you scale up workers that are not included in the network defined by the networking.machineNetwork[].cidr field from the installation configuration, you must add them to this list to prevent connection issues.

   This field is ignored if neither the httpProxy nor the httpsProxy field is set.

3. Save the manifest as migration-controller.yaml.

4. Apply the updated manifest:

   $ oc replace -f migration-controller.yaml -n openshift-migration

Migrating an application from the command line

You can migrate an application from the command line by using the Migration Toolkit for Containers (MTC) API.

Procedure

1. Create a MigCluster CR manifest for the host cluster:

   $ cat << EOF | oc apply -f -
   apiVersion: migration.openshift.io/v1alpha1
   kind: MigCluster
   metadata:
     name: <host_cluster>
     namespace: openshift-migration
   spec:
     isHostCluster: true
   EOF

2. Create a Secret CR manifest for each remote cluster:

   $ cat << EOF | oc apply -f -
   apiVersion: v1
   kind: Secret
   metadata:
     name: <cluster_secret>
     namespace: openshift-config
   type: Opaque
   data:
     saToken: <sa_token> (1)
   EOF

   1	Specify the base64-encoded migration-controller service account (SA) token of the remote cluster. You can obtain the token by running the following command:

   $ oc sa get-token migration-controller -n openshift-migration | base64 -w 0

3. Create a MigCluster CR manifest for each remote cluster:

   $ cat << EOF | oc apply -f -
   apiVersion: migration.openshift.io/v1alpha1
   kind: MigCluster
   metadata:
     name: <remote_cluster> (1)
     namespace: openshift-migration
   spec:
     exposedRegistryPath: <exposed_registry_route> (2)
     insecure: false (3)
     isHostCluster: false
     serviceAccountSecretRef:
       name: <remote_cluster_secret> (4)
       namespace: openshift-config
     url: <remote_cluster_url> (5)
   EOF

   1	Specify the Cluster CR of the remote cluster.
   2	Optional: For direct image migration, specify the exposed registry route.
   3	SSL verification is enabled if false. CA certificates are not required or checked if true.
   4	Specify the Secret CR of the remote cluster.
   5	Specify the URL of the remote cluster.

4. Verify that all clusters are in a Ready state:

   $ oc describe cluster <cluster>

5. Create a Secret CR manifest for the replication repository:

   $ cat << EOF | oc apply -f -
   apiVersion: v1
   kind: Secret
   metadata:
     namespace: openshift-config
     name: <migstorage_creds>
   type: Opaque
   data:
     aws-access-key-id: <key_id_base64> (1)
     aws-secret-access-key: <secret_key_base64> (2)
   EOF

   1	Specify the key ID in base64 format.
   2	Specify the secret key in base64 format.

   AWS credentials are base64-encoded by default. For other storage providers, you must encode your credentials by running the following command with each key:

   $ echo -n "<key>" | base64 -w 0 (1)

   1	Specify the key ID or the secret key. Both keys must be base64-encoded.

6. Create a MigStorage CR manifest for the replication repository:

   $ cat << EOF | oc apply -f -
   apiVersion: migration.openshift.io/v1alpha1
   kind: MigStorage
   metadata:
     name: <migstorage>
     namespace: openshift-migration
   spec:
     backupStorageConfig:
       awsBucketName: <bucket> (1)
       credsSecretRef:
         name: <storage_secret> (2)
         namespace: openshift-config
     backupStorageProvider: <storage_provider> (3)
     volumeSnapshotConfig:
       credsSecretRef:
         name: <storage_secret> (4)
         namespace: openshift-config
     volumeSnapshotProvider: <storage_provider> (5)
   EOF

   1	Specify the bucket name.
   2	Specify the Secrets CR of the object storage. You must ensure that the credentials stored in the Secrets CR of the object storage are correct.
   3	Specify the storage provider.
   4	Optional: If you are copying data by using snapshots, specify the Secrets CR of the object storage. You must ensure that the credentials stored in the Secrets CR of the object storage are correct.
   5	Optional: If you are copying data by using snapshots, specify the storage provider.

7. Verify that the MigStorage CR is in a Ready state:

   $ oc describe migstorage <migstorage>

8. Create a MigPlan CR manifest:

   $ cat << EOF | oc apply -f -
   apiVersion: migration.openshift.io/v1alpha1
   kind: MigPlan
   metadata:
     name: <migplan>
     namespace: openshift-migration
   spec:
     destMigClusterRef:
       name: <host_cluster>
       namespace: openshift-migration
     indirectImageMigration: true (1)
     indirectVolumeMigration: true (2)
     migStorageRef:
       name: <migstorage> (3)
       namespace: openshift-migration
     namespaces:
       - <source_namespace_1> (4)
       - <source_namespace_2>
       - <source_namespace_3>:<destination_namespace> (5)
     srcMigClusterRef:
       name: <remote_cluster> (6)
       namespace: openshift-migration
   EOF

   1	Direct image migration is enabled if false.
   2	Direct volume migration is enabled if false.
   3	Specify the name of the MigStorage CR instance.
   4	Specify one or more source namespaces. By default, the destination namespace has the same name.
   5	Specify a destination namespace if it is different from the source namespace.
   6	Specify the name of the source cluster MigCluster instance.

9. Verify that the MigPlan instance is in a Ready state:

   $ oc describe migplan <migplan> -n openshift-migration

10. Create a MigMigration CR manifest to start the migration defined in the MigPlan instance:

    $ cat << EOF | oc apply -f -
    apiVersion: migration.openshift.io/v1alpha1
    kind: MigMigration
    metadata:
     name: <migmigration>
     namespace: openshift-migration
    spec:
     migPlanRef:
       name: <migplan> (1)
       namespace: openshift-migration
     quiescePods: true (2)
     stage: false (3)
     rollback: false (4)
    EOF

    1	Specify the MigPlan CR name.
    2	The pods on the source cluster are stopped before migration if true.
    3	A stage migration, which copies most of the data without stopping the application, is performed if true.
    4	A completed migration is rolled back if true.

11. Verify the migration by watching the MigMigration CR progress:

    $ oc watch migmigration <migmigration> -n openshift-migration

    The output resembles the following:

    Example output

    Name:         c8b034c0-6567-11eb-9a4f-0bc004db0fbc
    Namespace:    openshift-migration
    Labels:       migration.openshift.io/migplan-name=django
    Annotations:  openshift.io/touch: e99f9083-6567-11eb-8420-0a580a81020c
    API Version:  migration.openshift.io/v1alpha1
    Kind:         MigMigration
    ...
    Spec:
     Mig Plan Ref:
       Name:       migplan
       Namespace:  openshift-migration
     Stage:        false
    Status:
     Conditions:
       Category:              Advisory
       Last Transition Time:  2021-02-02T15:04:09Z
       Message:               Step: 19/47
       Reason:                InitialBackupCreated
       Status:                True
       Type:                  Running
       Category:              Required
       Last Transition Time:  2021-02-02T15:03:19Z
       Message:               The migration is ready.
       Status:                True
       Type:                  Ready
       Category:              Required
       Durable:               true
       Last Transition Time:  2021-02-02T15:04:05Z
       Message:               The migration registries are healthy.
       Status:                True
       Type:                  RegistriesHealthy
     Itinerary:               Final
     Observed Digest:         7fae9d21f15979c71ddc7dd075cb97061895caac5b936d92fae967019ab616d5
     Phase:                   InitialBackupCreated
     Pipeline:
       Completed:  2021-02-02T15:04:07Z
       Message:    Completed
       Name:       Prepare
       Started:    2021-02-02T15:03:18Z
       Message:    Waiting for initial Velero backup to complete.
       Name:       Backup
       Phase:      InitialBackupCreated
       Progress:
         Backup openshift-migration/c8b034c0-6567-11eb-9a4f-0bc004db0fbc-wpc44: 0 out of estimated total of 0 objects backed up (5s)
       Started:        2021-02-02T15:04:07Z
       Message:        Not started
       Name:           StageBackup
       Message:        Not started
       Name:           StageRestore
       Message:        Not started
       Name:           DirectImage
       Message:        Not started
       Name:           DirectVolume
       Message:        Not started
       Name:           Restore
       Message:        Not started
       Name:           Cleanup
     Start Timestamp:  2021-02-02T15:03:18Z
    Events:
     Type    Reason   Age                 From                     Message
     ----    ------   ----                ----                     -------
     Normal  Running  57s                 migmigration_controller  Step: 2/47
     Normal  Running  57s                 migmigration_controller  Step: 3/47
     Normal  Running  57s (x3 over 57s)   migmigration_controller  Step: 4/47
     Normal  Running  54s                 migmigration_controller  Step: 5/47
     Normal  Running  54s                 migmigration_controller  Step: 6/47
     Normal  Running  52s (x2 over 53s)   migmigration_controller  Step: 7/47
     Normal  Running  51s (x2 over 51s)   migmigration_controller  Step: 8/47
     Normal  Ready    50s (x12 over 57s)  migmigration_controller  The migration is ready.
     Normal  Running  50s                 migmigration_controller  Step: 9/47
     Normal  Running  50s                 migmigration_controller  Step: 10/47

Mapping destination namespaces in the MigPlan custom resource

If you map destination namespaces in the MigPlan custom resource (CR), you must ensure that the namespaces are not duplicated on the source or the destination clusters because the UID and GID ranges of the namespaces are copied during migration.

Two source namespaces mapped to the same destination namespace

spec:
  namespaces:
    - namespace_2
    - namespace_1:namespace_2

If you want the source namespace to be mapped to a namespace of the same name, you do not need to create a mapping. By default, a source namespace and a target namespace have the same name.

Incorrect namespace mapping

spec:
  namespaces:
    - namespace_1:namespace_1

Correct namespace reference

spec:
  namespaces:
    - namespace_1

Migrating an application’s state

You can perform repeatable, state-only migrations by selecting specific persistent volume claims (PVCs). During a state migration, Migration Toolkit for Containers (MTC) copies persistent volume (PV) data to the target cluster. PV references are not moved. The application pods continue to run on the source cluster.

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

You can use state migration to migrate namespaces within the same cluster.

You can migrate PV data from the source cluster to PVCs that are already provisioned in the target cluster by mapping PVCs in the MigPlan CR. This ensures that the target PVCs of migrated applications are synchronized with the source PVCs.

You can perform a one-time migration of Kubernetes objects that store application state.

Excluding persistent volume claims

You can exclude persistent volume claims (PVCs) by adding the spec.persistentVolumes.pvc.selection.action parameter to the MigPlan custom resource (CR) after the persistent volumes (PVs) have been discovered.

Prerequisites

• MigPlan CR with discovered PVs.

Procedure

• Add the spec.persistentVolumes.pvc.selection.action parameter to the MigPlan CR and set its value to skip:

  apiVersion: migration.openshift.io/v1alpha1
  kind: MigPlan
  metadata:
    name: <migplan>
    namespace: openshift-migration
  spec:
  ...
    persistentVolumes:
    - capacity: 10Gi
      name: <pv_name>
      pvc:
  ...
      selection:
        action: skip (1)

  1	skip excludes the PVC from the migration plan.

Mapping persistent volume claims

You can map persistent volume claims (PVCs) by updating the spec.persistentVolumes.pvc.name parameter in the MigPlan custom resource (CR) after the persistent volumes (PVs) have been discovered.

Prerequisites

• MigPlan CR with discovered PVs.

Procedure

• Update the spec.persistentVolumes.pvc.name parameter in the MigPlan CR:

  apiVersion: migration.openshift.io/v1alpha1
  kind: MigPlan
  metadata:
    name: <migplan>
    namespace: openshift-migration
  spec:
  ...
    persistentVolumes:
    - capacity: 10Gi
      name: <pv_name>
      pvc:
        name: <source_pvc>:<destination_pvc> (1)

  1	Specify the PVC on the source cluster and the PVC on the destination cluster. If the destination PVC does not exist, it will be created. You can use this mapping to change the PVC name during migration.

Migrating Kubernetes objects

You can perform a one-time migration of Kubernetes objects that constitute an application’s state.

You add Kubernetes objects to the MigPlan CR by using the following options:

• Adding the Kubernetes objects to the includedResources section.

• Using the labelSelector parameter to reference labeled Kubernetes objects.

If you set both parameters, the label is used to filter the included resources, for example, to migrate Secret and ConfigMap resources with the label app: frontend.

Procedure

• Update the MigPlan CR:

  apiVersion: migration.openshift.io/v1alpha1
  kind: MigPlan
  metadata:
    name: <migplan>
    namespace: openshift-migration
  spec:
    includedResources: (1)
    - kind: <Secret>
      group: ""
    - kind: <ConfigMap>
      group: ""
  ...
    labelSelector:
      matchLabels:
        <app: frontend> (2)

  1	Specify the kind and group of each resource.
  2	Specify the label of the resources to migrate.

Migration hooks

You can add up to four migration hooks to a single migration plan, with each hook running at a different phase of the migration. Migration hooks perform tasks such as customizing application quiescence, manually migrating unsupported data types, and updating applications after migration.

A migration hook runs on a source or a target cluster at one of the following migration steps:

• PreBackup: Before resources are backed up on the source cluster.

• PostBackup: After resources are backed up on the source cluster.

• PreRestore: Before resources are restored on the target cluster.

• PostRestore: After resources are restored on the target cluster.

You can create a hook by creating an Ansible playbook that runs with the default Ansible image or with a custom hook container.

Ansible playbook

The Ansible playbook is mounted on a hook container as a config map. The hook container runs as a job, using the cluster, service account, and namespace specified in the MigPlan custom resource. The job continues to run until it reaches the default limit of 6 retries or a successful completion. This continues even if the initial pod is evicted or killed.

The default Ansible runtime image is registry.redhat.io/rhmtc/openshift-migration-hook-runner-rhel7:1.6. This image is based on the Ansible Runner image and includes python-openshift for Ansible Kubernetes resources and an updated oc binary.

Custom hook container

You can use a custom hook container instead of the default Ansible image.

Writing an Ansible playbook for a migration hook

You can write an Ansible playbook to use as a migration hook. The hook is added to a migration plan by using the MTC web console or by specifying values for the spec.hooks parameters in the MigPlan custom resource (CR) manifest.

The Ansible playbook is mounted onto a hook container as a config map. The hook container runs as a job, using the cluster, service account, and namespace specified in the MigPlan CR. The hook container uses a specified service account token so that the tasks do not require authentication before they run in the cluster.

Ansible modules

You can use the Ansible shell module to run oc commands.

Example shell module

- hosts: localhost
  gather_facts: false
  tasks:
  - name: get pod name
    shell: oc get po --all-namespaces

You can use kubernetes.core modules, such as k8s_info, to interact with Kubernetes resources.

Example k8s_facts module

- hosts: localhost
  gather_facts: false
  tasks:
  - name: Get pod
    k8s_info:
      kind: pods
      api: v1
      namespace: openshift-migration
      name: "{{ lookup( 'env', 'HOSTNAME') }}"
    register: pods
  - name: Print pod name
    debug:
      msg: "{{ pods.resources[0].metadata.name }}"

You can use the fail module to produce a non-zero exit status in cases where a non-zero exit status would not normally be produced, ensuring that the success or failure of a hook is detected. Hooks run as jobs and the success or failure status of a hook is based on the exit status of the job container.

Example fail module

- hosts: localhost
  gather_facts: false
  tasks:
  - name: Set a boolean
    set_fact:
      do_fail: true
  - name: "fail"
    fail:
      msg: "Cause a failure"
    when: do_fail

Environment variables

The MigPlan CR name and migration namespaces are passed as environment variables to the hook container. These variables are accessed by using the lookup plug-in.

Example environment variables

- hosts: localhost
  gather_facts: false
  tasks:
  - set_fact:
      namespaces: "{{ (lookup( 'env', 'migration_namespaces')).split(',') }}"
  - debug:
      msg: "{{ item }}"
    with_items: "{{ namespaces }}"
  - debug:
      msg: "{{ lookup( 'env', 'migplan_name') }}"

Configuration options

You can configure the following options for the MigPlan and MigrationController custom resources (CRs) to perform large-scale migrations and to improve performance.

Increasing limits for large migrations

You can increase the limits on migration objects and container resources for large migrations with the Migration Toolkit for Containers (MTC).

Procedure

1. Edit the MigrationController custom resource (CR) manifest:

   $ oc edit migrationcontroller -n openshift-migration

2. Update the following parameters:

   ...
   mig_controller_limits_cpu: "1" (1)
   mig_controller_limits_memory: "10Gi" (2)
   ...
   mig_controller_requests_cpu: "100m" (3)
   mig_controller_requests_memory: "350Mi" (4)
   ...
   mig_pv_limit: 100 (5)
   mig_pod_limit: 100 (6)
   mig_namespace_limit: 10 (7)
   ...

   1	Specifies the number of CPUs available to the MigrationController CR.
   2	Specifies the amount of memory available to the MigrationController CR.
   3	Specifies the number of CPU units available for MigrationController CR requests. 100m represents 0.1 CPU units (100 * 1e-3).
   4	Specifies the amount of memory available for MigrationController CR requests.
   5	Specifies the number of persistent volumes that can be migrated.
   6	Specifies the number of pods that can be migrated.
   7	Specifies the number of namespaces that can be migrated.

3. Create a migration plan that uses the updated parameters to verify the changes.

   If your migration plan exceeds the MigrationController CR limits, the MTC console displays a warning message when you save the migration plan.

Excluding resources from a migration plan

You can exclude resources, for example, image streams, persistent volumes (PVs), or subscriptions, from a Migration Toolkit for Containers (MTC) migration plan to reduce the resource load for migration or to migrate images or PVs with a different tool.

By default, the MTC excludes service catalog resources and Operator Lifecycle Manager (OLM) resources from migration. These resources are parts of the service catalog API group and the OLM API group, neither of which is supported for migration at this time.

Procedure

1. Edit the MigrationController custom resource manifest:

   $ oc edit migrationcontroller <migration_controller> -n openshift-migration

2. Update the spec section by adding a parameter to exclude specific resources or by adding a resource to the excluded_resources parameter if it does not have its own exclusion parameter:

   apiVersion: migration.openshift.io/v1alpha1
   kind: MigrationController
   metadata:
     name: migration-controller
     namespace: openshift-migration
   spec:
     disable_image_migration: true (1)
     disable_pv_migration: true (2)
     ...
     excluded_resources: (3)
     - imagetags
     - templateinstances
     - clusterserviceversions
     - packagemanifests
     - subscriptions
     - servicebrokers
     - servicebindings
     - serviceclasses
     - serviceinstances
     - serviceplans
     - operatorgroups
     - events
     - events.events.k8s.io

   1	Add disable_image_migration: true to exclude image streams from the migration. Do not edit the excluded_resources parameter. imagestreams is added to excluded_resources when the MigrationController pod restarts.
   2	Add disable_pv_migration: true to exclude PVs from the migration plan. Do not edit the excluded_resources parameter. persistentvolumes and persistentvolumeclaims are added to excluded_resources when the MigrationController pod restarts. Disabling PV migration also disables PV discovery when you create the migration plan.
   3	You can add OKD resources to the excluded_resources list. Do not delete the default excluded resources. These resources are problematic to migrate and must be excluded.

3. Wait two minutes for the MigrationController pod to restart so that the changes are applied.

4. Verify that the resource is excluded:

   $ oc get deployment -n openshift-migration migration-controller -o yaml | grep EXCLUDED_RESOURCES -A1

   The output contains the excluded resources:

   Example output

       - name: EXCLUDED_RESOURCES
         value:
         imagetags,templateinstances,clusterserviceversions,packagemanifests,subscriptions,servicebrokers,servicebindings,serviceclasses,serviceinstances,serviceplans,imagestreams,persistentvolumes,persistentvolumeclaims

Enabling persistent volume resizing for direct volume migration

You can enable persistent volume (PV) resizing for direct volume migration to avoid running out of disk space on the destination cluster.

When the disk usage of a PV reaches a configured level, the MigrationController custom resource (CR) compares the requested storage capacity of a persistent volume claim (PVC) to its actual provisioned capacity. Then, it calculates the space required on the destination cluster.

A pv_resizing_threshold parameter determines when PV resizing is used. The default threshold is 3%. This means that PV resizing occurs when the disk usage of a PV is more than 97%. You can increase this threshold so that PV resizing occurs at a lower disk usage level.

PVC capacity is calculated according to the following criteria:

• If the requested storage capacity (spec.resources.requests.storage) of the PVC is not equal to its actual provisioned capacity (status.capacity.storage), the greater value is used.

• If a PV is provisioned through a PVC and then subsequently changed so that its PV and PVC capacities no longer match, the greater value is used.

Prerequisites

• The PVCs must be attached to one or more running pods so that the MigrationController CR can execute commands.

Procedure

1. Log in to the host cluster.

2. Enable PV resizing by patching the MigrationController CR:

   $ oc patch migrationcontroller migration-controller -p '{"spec":{"enable_dvm_pv_resizing":true}}' \ (1)
     --type='merge' -n openshift-migration

   1	Set the value to false to disable PV resizing.

3. Optional: Update the pv_resizing_threshold parameter to increase the threshold:

   $ oc patch migrationcontroller migration-controller -p '{"spec":{"pv_resizing_threshold":41}}' \ (1)
     --type='merge' -n openshift-migration

   1	The default value is 3.

   When the threshold is exceeded, the following status message is displayed in the MigPlan CR status:

   status:
     conditions:
   ...
     - category: Warn
       durable: true
       lastTransitionTime: "2021-06-17T08:57:01Z"
       message: 'Capacity of the following volumes will be automatically adjusted to avoid disk capacity issues in the target cluster:  [pvc-b800eb7b-cf3b-11eb-a3f7-0eae3e0555f3]'
       reason: Done
       status: "False"
       type: PvCapacityAdjustmentRequired

   For AWS gp2 storage, this message does not appear unless the pv_resizing_threshold is 42% or greater because of the way gp2 calculates volume usage and size. (BZ#1973148)

Enabling cached Kubernetes clients

You can enable cached Kubernetes clients in the MigrationController custom resource (CR) for improved performance during migration. The greatest performance benefit is displayed when migrating between clusters in different regions or with significant network latency.

Cached clients require extra memory because the MigrationController CR caches all API resources that are required for interacting with MigCluster CRs. Requests that are normally sent to the API server are directed to the cache instead. The cache watches the API server for updates.

You can increase the memory limits and requests of the MigrationController CR if OOMKilled errors occur after you enable cached clients.

Procedure

1. Enable cached clients by running the following command:

   $ oc -n openshift-migration patch migrationcontroller migration-controller --type=json --patch \
     '[{ "op": "replace", "path": "/spec/mig_controller_enable_cache", "value": true}]'

2. Optional: Increase the MigrationController CR memory limits by running the following command:

   $ oc -n openshift-migration patch migrationcontroller migration-controller --type=json --patch \
     '[{ "op": "replace", "path": "/spec/mig_controller_limits_memory", "value": <10Gi>}]'

3. Optional: Increase the MigrationController CR memory requests by running the following command:

   $ oc -n openshift-migration patch migrationcontroller migration-controller --type=json --patch \
     '[{ "op": "replace", "path": "/spec/mig_controller_requests_memory", "value": <350Mi>}]'