Chapter 13. Migrating virtual machine instances between Compute nodes Warning The content for this feature is available in this release as a Documentation Preview , and therefore is not fully verified by Red Hat. Use it only for testing, and do not use in a production environment. You sometimes need to migrate instances from one Compute node to another Compute node in the data plane, to perform maintenance, rebalance the workload, or replace a failed or failing node. Compute node maintenance If you need to temporarily take a Compute node out of service, for instance, to perform hardware maintenance or repair, kernel upgrades and software updates, you can migrate instances running on the Compute node to another Compute node. Failing Compute node If a Compute node is about to fail and you need to service it or replace it, you can migrate instances from the failing Compute node to a healthy Compute node. Failed Compute nodes If a Compute node has already failed, you can evacuate the instances. You can rebuild instances from the original image on another Compute node, using the same name, UUID, network addresses, and any other allocated resources the instance had before the Compute node failed. Workload rebalancing You can migrate one or more instances to another Compute node to rebalance the workload. For example, you can consolidate instances on a Compute node to conserve power, migrate instances to a Compute node that is physically closer to other networked resources to reduce latency, or distribute instances across Compute nodes to avoid hot spots and increase resiliency. All Compute nodes provide secure migration. All Compute nodes also require a shared SSH key to provide the users of each host with access to other Compute nodes during the migration process. 13.1. Migration types Red Hat OpenStack Services on OpenShift (RHOSO) supports the following types of migration. Cold migration Cold migration, or non-live migration, involves shutting down a running instance before migrating it from the source Compute node to the destination Compute node. Cold migration involves some downtime for the instance. The migrated instance maintains access to the same volumes and IP addresses. Note Cold migration requires that both the source and destination Compute nodes are running. Live migration Live migration involves moving the instance from the source Compute node to the destination Compute node without shutting it down, and while maintaining state consistency. Live migrating an instance involves little or no perceptible downtime. However, live migration does impact performance for the duration of the migration operation. Therefore, instances should be taken out of the critical path while being migrated. Important Live migration impacts the performance of the workload being moved. Red Hat does not provide support for increased packet loss, network latency, memory latency or a reduction in network bandwith, memory bandwidth, storage IO, or CPU peformance during live migration. Note Live migration requires that both the source and destination Compute nodes are running. Evacuation If you need to migrate instances because the source Compute node has already failed, you can evacuate the instances. 13.2. Migration constraints Migration constraints typically arise with block migration, configuration disks, or when one or more instances access physical hardware on the Compute node. CPU constraints The source and destination Compute nodes must have the same CPU architecture. For example, Red Hat does not support migrating an instance from a ppc64le CPU to a x86_64 CPU. Migration between different CPU models is not supported. In some cases, the CPU of the source and destination Compute node must match exactly, such as instances that use CPU host passthrough. In all cases, the CPU features of the destination node must be a superset of the CPU features on the source node. Memory constraints The destination Compute node must have sufficient available RAM. Memory oversubscription can cause migration to fail. Block migration constraints Migrating instances that use disks that are stored locally on a Compute node takes significantly longer than migrating volume-backed instances that use shared storage, such as Red Hat Ceph Storage. This latency arises because OpenStack Compute (nova) migrates local disks block-by-block between the Compute nodes over the control plane network by default. By contrast, volume-backed instances that use shared storage, such as Red Hat Ceph Storage, do not have to migrate the volumes, because each Compute node already has access to the shared storage. Note Network congestion in the control plane network caused by migrating local disks or instances that consume large amounts of RAM might impact the performance of other systems that use the control plane network, such as RabbitMQ. Read-only drive migration constraints Migrating a drive is supported only if the drive has both read and write capabilities. For example, OpenStack Compute (nova) cannot migrate a CD-ROM drive or a read-only config drive. However, OpenStack Compute (nova) can migrate a drive with both read and write capabilities. The config drive vfat is the only config drive format that OpenStack Compute (nova) can migrate. Live migration constraints In some cases, live migrating instances involves additional constraints. Important Live migration impacts the performance of the workload being moved. Red Hat does not provide support for increased packet loss, network latency, memory latency or a reduction in network bandwidth, memory bandwidth, storage IO, or CPU performance during live migration. No new operations during migration To achieve state consistency between the copies of the instance on the source and destination nodes, RHOSP must prevent new operations during live migration. Otherwise, live migration might take a long time or potentially never end if writes to memory occur faster than live migration can replicate the state of the memory. CPU pinning with NUMA The NovaSchedulerEnabledFilters parameter in the Compute configuration must include the values AggregateInstanceExtraSpecsFilter and NUMATopologyFilter . Multi-cell clouds In a multi-cell cloud, you can live migrate instances to a different host in the same cell, but not across cells. Floating instances When you migrate shared (floating) instances, the value of the NovaComputeCpuSharedSet field between the destination and source Compute nodes must match, so that the instances are allocated to CPUs configured for shared (unpinned) instances at the destination. Therefore, if you need to live migrate floating instances, ensure that all the Compute nodes have the same CPU mappings for dedicated (pinned) and shared instances, or use a host aggregate for the shared instances. Destination Compute node capacity The destination Compute node must have sufficient capacity to host the instance that you want to migrate. SR-IOV live migration Instances with SR-IOV-based network interfaces can be live migrated. Live migrating instances with direct mode SR-IOV network interfaces incurs network downtime. This is because the direct mode interfaces need to be detached and re-attached during the migration. Live migration on ML2/OVS deployments During the live migration process, when the virtual machine is unpaused in the destination host, the metadata service might not be available because the metadata server proxy has not yet spawned. This unavailability is brief. The service becomes available momentarily and the live migration succeeds. Constraints that preclude live migration You cannot live migrate an instance that uses the following features. PCI passthrough QEMU/KVM hypervisors support attaching PCI devices on the Compute node to an instance. Use PCI passthrough to give an instance exclusive access to PCI devices, which appear and behave as if they are physically attached to the operating system of the instance. However, because PCI passthrough involves direct access to the physical devices, QEMU/KVM does not support live migration of instances using PCI passthrough. 13.3. Preparing to migrate Before you migrate one or more instances, you need to determine the Compute node names and the IDs of the instances to migrate. Prerequisites You are logged on to a workstation that has access to the RHOSO control plane as a user with cluster-admin privileges. The oc command line tool is installed on the workstation. Procedure Access the remote shell for the OpenStackClient pod from your workstation: List the instances on the source Compute node and locate the ID of the instance or instances that you want to migrate: Replace <source> with the name or ID of the source Compute node. Optional: If you are migrating instances from a source Compute node to perform maintenance on the node, you must disable the node to prevent the scheduler from assigning new instances to the node during maintenance: Replace <source> with the host name of the source Compute node. Exit the OpenStackClient pod: You are now ready to perform the migration. Follow the required procedure detailed in Cold migrating an instance or Live migrating an instance . 13.4. Cold migrating an instance Cold migrating an instance involves stopping the instance and moving it to another Compute node. Cold migration facilitates migration scenarios that live migrating cannot facilitate, such as migrating instances that use PCI passthrough. The scheduler automatically selects the destination Compute node. For more information, see Migration constraints . Procedure Access the remote shell for the OpenStackClient pod from your workstation: To cold migrate an instance, enter the following command to power off and move the instance: Replace <instance> with the name or ID of the instance to migrate. Specify the --block-migration flag if migrating a locally stored volume. Wait for migration to complete. While you wait for the instance migration to complete, you can check the migration status. For more information, see Checking migration status . Check the status of the instance: A status of "VERIFY_RESIZE" indicates you need to confirm or revert the migration: If the migration worked as expected, confirm it: Replace <instance> with the name or ID of the instance to migrate. A status of "ACTIVE" indicates that the instance is ready to use. If the migration did not work as expected, revert it: Replace <instance> with the name or ID of the instance. Restart the instance: Replace <instance> with the name or ID of the instance. Optional: If you disabled the source Compute node for maintenance, you must re-enable the node so that new instances can be assigned to it: Replace <source> with the host name of the source Compute node. Exit the OpenStackClient pod: 13.5. Live migrating an instance Live migration moves an instance from a source Compute node to a destination Compute node with a minimal amount of downtime. Live migration might not be appropriate for all instances. For more information, see Migration constraints . Procedure Access the remote shell for the OpenStackClient pod from your workstation: To live migrate an instance, specify the instance and the destination Compute node: Replace <instance> with the name or ID of the instance. Replace <dest> with the name or ID of the destination Compute node. Note The openstack server migrate command covers migrating instances with shared storage, which is the default. Specify the --block-migration flag to migrate a locally stored volume: Confirm that the instance is migrating: Wait for migration to complete. While you wait for the instance migration to complete, you can check the migration status. For more information, see Checking migration status . Check the status of the instance to confirm if the migration was successful: Replace <dest> with the name or ID of the destination Compute node. Optional: If you disabled the source Compute node for maintenance, you must re-enable the node so that new instances can be assigned to it: Replace <source> with the host name of the source Compute node. Exit the OpenStackClient pod: 13.6. Checking migration status Migration involves several state transitions before migration is complete. During a healthy migration, the migration state typically transitions as follows: Queued: The Compute service has accepted the request to migrate an instance, and migration is pending. Preparing: The Compute service is preparing to migrate the instance. Running: The Compute service is migrating the instance. Post-migrating: The Compute service has built the instance on the destination Compute node and is releasing resources on the source Compute node. Completed: The Compute service has completed migrating the instance and finished releasing resources on the source Compute node. Procedure Access the remote shell for the OpenStackClient pod from your workstation: Retrieve the list of migration IDs for the instance: Replace <instance> with the name or ID of the instance. Show the status of the migration: Replace <instance> with the name or ID of the instance. Replace <migration_id> with the ID of the migration. Running the openstack server migration show command returns the following example output: Tip The Compute service measures progress of the migration by the number of remaining memory bytes to copy. If this number does not decrease over time, the migration might be unable to complete, and the Compute service might abort it. Exit the OpenStackClient pod: Sometimes instance migration can take a long time or encounter errors. For more information, see Troubleshooting migration . 13.7. Evacuating an instance If you want to move an instance from a failed or shut-down Compute node to a new host in the same environment, you can evacuate it. The evacuate process destroys the original instance and rebuilds it on another Compute node using the original image, instance name, UUID, network addresses, and any other resources the original instance had allocated to it. If the instance uses shared storage, the instance root disk is not rebuilt during the evacuate process, as the disk remains accessible by the destination Compute node. If the instance does not use shared storage, then the instance root disk is also rebuilt on the destination Compute node. Note You can only perform an evacuation when the Compute node is fenced, and the API reports that the state of the Compute node is "down" or "forced-down". If the Compute node is not reported as "down" or "forced-down", the evacuate command fails. To perform an evacuation, you must be a cloud administrator. 13.7.1. Evacuating an instance To evacuate all instances on a host, you must evacuate them one at a time. Procedure Access the remote shell for the OpenStackClient pod from your workstation: Confirm that the instance is not running: Replace <node> with the name or UUID of the Compute node that hosts the instance. Check the instance task state: Replace <instance> with the name or UUID of the instance that you want to evacuate. Note If the instance task state is not "NONE" the evacuation might fail. Confirm that the host Compute node is fenced or shut down: Replace <node> with the name or UUID of the Compute node that hosts the instance to evacuate. To perform an evacuation, the Compute node must have a status of down or forced-down . Disable the Compute node: Replace <node> with the name of the Compute node to evacuate the instance from. Replace <disable_host_reason> with details about why you disabled the Compute node. Evacuate the instance: Optional: Replace <dest> with the name of the Compute node to evacuate the instance to. If you do not specify the destination Compute node, the Compute scheduler selects one for you. You can find possible Compute nodes by using the following command: Optional: Replace <password> with the administrative password required to access the evacuated instance. If a password is not specified, a random password is generated and output when the evacuation is complete. Note The password is changed only when ephemeral instance disks are stored on the local hypervisor disk. The password is not changed if the instance is hosted on shared storage or has a Block Storage volume attached, and no error message is displayed to inform you that the password was not changed. Replace <instance> with the name or ID of the instance to evacuate. Note If the evacuation fails and the task state of the instance is not "NONE", contact Red Hat Support for help to recover the instance. Optional: Enable the Compute node when it is recovered: Replace <node> with the name of the Compute node to enable. Exit the OpenStackClient pod: 13.8. Troubleshooting migration The following issues can arise during instance migration: The migration process encounters errors. The migration process never ends. Performance of the instance degrades after migration. 13.8.1. Errors during migration The following issues can send the migration operation into an error state: The Compute service is shutting down. A race condition occurs. When live migration enters a failed state, it is typically followed by an error state. The following common issues can cause a failed state: A destination Compute host is not available. A scheduler exception occurs. The rebuild process fails due to insufficient computing resources. A server group check fails. The instance on the source Compute node gets deleted before migration to the destination Compute node is complete. 13.8.2. Never-ending live migration Live migrations are left in a perpetual running state when they fail to complete, which can occur when requests from the guest OS to the instance running on the source Compute node create changes that occur faster than the Compute service can replicate them to the destination Compute node. Use one of the following methods to address this situation: Abort the live migration. Force the live migration to complete. Aborting live migration If the instance state changes faster than the migration procedure can copy it to the destination node, and you do not want to temporarily suspend the instance operations, you can abort the live migration. Procedure Access the remote shell for the OpenStackClient pod from your workstation: Retrieve the list of migrations for the instance: Replace <instance> with the name or ID of the instance. Abort the live migration: Replace <instance> with the name or ID of the instance. Replace <migration_id> with the ID of the migration. Exit the OpenStackClient pod: Forcing live migration to complete If the instance state changes faster than the migration procedure can copy it to the destination node, and you want to temporarily suspend the instance operations to force migration to complete, you can force the live migration procedure to complete. Important Forcing live migration to complete might lead to perceptible downtime. Procedure Access the remote shell for the OpenStackClient pod from your workstation: Retrieve the list of migrations for the instance: Replace <instance> with the name or ID of the instance. Force the live migration to complete: Replace <instance> with the name or ID of the instance. Replace <migration_id> with the ID of the migration. Exit the OpenStackClient pod:

Commands:
oc rsh -n openstack openstackclient
openstack server list --host <source> --all-projects
openstack compute service set <source> nova-compute --disable
exit
oc rsh -n openstack openstackclient
openstack server migrate <instance> --wait
openstack server list --all-projects
openstack server resize --confirm <instance>
openstack server resize --revert <instance>
openstack server start <instance>
openstack compute service set <source> nova-compute --enable
exit
oc rsh -n openstack openstackclient
openstack server migrate <instance> --live-migration [--host <dest>] --wait
openstack server migrate <instance> --live-migration [--host <dest>] --wait --block-migration
openstack server show <instance> +----------------------+--------------------------------------+ | Field | Value | +----------------------+--------------------------------------+ | ... | ... | | status | MIGRATING | | ... | ... | +----------------------+--------------------------------------+
openstack server list --host <dest> --all-projects
openstack compute service set <source> nova-compute --enable
exit
oc rsh -n openstack openstackclient
openstack server migration list --server <instance> +----+-------------+----------- (...) | Id | Source Node | Dest Node | (...) +----+-------------+-----------+ (...) | 2 | - | - | (...) +----+-------------+-----------+ (...)
openstack server migration show <instance> <migration_id>
+------------------------+--------------------------------------+ | Property | Value | +------------------------+--------------------------------------+ | created_at | 2017-03-08T02:53:06.000000 | | dest_compute | controller | | dest_host | - | | dest_node | - | | disk_processed_bytes | 0 | | disk_remaining_bytes | 0 | | disk_total_bytes | 0 | | id | 2 | | memory_processed_bytes | 65502513 | | memory_remaining_bytes | 786427904 | | memory_total_bytes | 1091379200 | | server_uuid | d1df1b5a-70c4-4fed-98b7-423362f2c47c | | source_compute | compute2 | | source_node | - | | status | running | | updated_at | 2017-03-08T02:53:47.000000 | +------------------------+--------------------------------------+
exit
oc rsh -n openstack openstackclient
openstack server list --host <node> --all-projects
openstack server show <instance> +----------------------+--------------------------------------+ | Field | Value | +----------------------+--------------------------------------+ | ... | ... | | status | NONE | | ... | ... | +----------------------+--------------------------------------+
openstack baremetal node show <node>
openstack compute service set <node> nova-compute --disable --disable-reason <disable_host_reason>
openstack server evacuate [--host <dest>] [--password <password>] <instance>
openstack hypervisor list
openstack compute service set <node> nova-compute --enable
exit
oc rsh -n openstack openstackclient
openstack server migration list --server <instance>
openstack server migration abort <instance> <migration_id>
exit
oc rsh -n openstack openstackclient
openstack server migration list --server <instance>
openstack server migration force complete <instance> <migration_id>
exit