- Cold migration -- migration of a VM which requires the VM to be powered off during the migrate operation during which time the VM is inaccessible.
- Hot or live migration -- zero down-time migration whereupon the VM is not powered off during the migration and thus remains accessible.
Understanding these VM migration operations from an OpenStack internals perspective can be a daunting task. I had the pleasure of digging into these flows in the latter part of 2013 and as part of that effort created a rough outline of the internal flows. Other's I've worked with found these flow outlines useful and thus they're provided below.
Note -- The outlines below were created based on the OpenStack source in late 2013 and thus reflect the state of OpenStack at that point in time.
Live Migration Flow:
- nova.api.openstack.compute.contrib.admin_actions._migrate_live()
- nova.compute.api.live_migrate()
- update instance state to MIGRATING state
- call into scheduler to live migrate (scheduler hint will be set to the host select (which may be none)).
- nova.scheduler.manager.live_migration()
- nova.scheduler.manager._schedule_live_migration()
- nova.conductor.tasks.live_migrate.LiveMigrationTask.execute()
- check that the instance is running
- check that the instance's host is up
- if destination host provided, check that it..
- is different than the instance's host
- is up
- has enough memory
- is compatible with the instance's host (i.e. hypervisor type and version)
- passes live migration checks (call using amqp rpc into nova manager check_can_live_migrate_destination)
- else destination host not provided, find a candidate destination host and check that it...
- is compatible with the instance's host (i.e. hypervisor type and version)
- passes live migration checks (call using amqp rpc into nova manager check_can_live_migrate_destination)
- call using amqp rpc into nova manager live_migration
Note: Migration data is initially set by check_can_live_migrate_destination and can be used for implementation specific parameters from this point.
- nova.compute.manager.check_can_live_migrate_destination()
- driver.check_can_live_migrate_destination()
- call using amqp rpc into nova manager check_can_live_migrate_source
- driver.check_can_live_migrate_destination_cleanup()
- nova.compute.manager.check_can_live_migrate_source()
- determine if the instance is volume backed and add result to the migration data
- driver.check_can_live_migrate_source()
- nova.compute.manager.live_migration()
- if block migration request then driver.get_instance_disk_info()
- call using amqp rpc into nova manager pre_live_migration
- Error handler: _rollback_live_migration
- driver.live_migration()
- nova.compute.manager.pre_live_migration()
- get the block device information for the instance
- get the network information for the instance
- driver.pre_live_migration()
- setup networks on destination host by calling the network API setup_networks_on_host
- driver.ensure_filtering_rules_for_instance()
- nova.compute.manager._rollback_live_migration()
- update instance state to ACTIVE state
- re-setup networks on source host by calling the network API setup_networks_on_host
- for each instance volume connection call using amqp rpc into nova manager remove_volume_connection
- if block migration or volume backed migration without shared storage
- call using amqp rpc into nova manager rollback_live_migration_at_destination
- nova.compute.manager._post_live_migration()
- driver.get_volume_connector()
- for each instance volume connection call the volume API terminate_connection
- driver.unfilter_instance()
- call into conductor to network_migrate_instance_start which will eventually call the network API migrate_instance_start
- call using amqp rpc into nova manager post_live_migration_at_destination
- if block migration or not shared storage driver.destroy()
- else driver.unplug_vifs()
- tear down networks on source host by calling the network API setup_networks_on_host
- nova.compute.manager.post_live_migration_at_destination()
- setup networks on destination host by calling the network API setup_networks_on_host
- call into conductor to network_migrate_instance_finish which will eventually call the network API migrate_instance_finish
- driver.post_live_migration_at_destination()
- update instance to ACTIVE state
- setup networks on destination host by calling the network API setup_networks_on_host
- nova.compute.manager.rollback_live_migration_at_destination()
- tear down networks on destination host by calling the network API setup_networks_on_host
- driver.destroy()
- nova.compute.manager.remove_volume_connection()
- call _detach_volume
- driver.get_volume_connector()
- remove the volume connection by calling the volume API terminate_connection
- nova.compute.manager._detach_volume()
- driver.detach_volume()
- Since the live migration failed the VM should not be on the destination host. So this should be a no-op.
- If there is an exception detaching the volume then rollback the detach by calling the volume API roll_detaching
- driver.detach_volume()
Cold Migration Flow:
- nova.api.openstack.compute.servers._resize()
- nova.api.openstack.compute.contrib.admin_actions._migrate()
- nova.compute.api.resize()
- if flavor_id is not passed, migrate host only and keep the original flavor
- else flavor_id is given, migrate host and resize to new flavor
- lookup the image for the instance by calling the image API show
- check quota headroom and reserve
- update instance to RESIZE_PREP state
- determine if the instance's current host should be ignored as a migration target and update filter properties for the scheduler accordingly
- call into scheduler to prep_resize
- nova.scheduler.manager.prep_resize()
- call scheduler driver to schedule_prep_resize
- if no valid host was found then update instance to ACTIVE state and rollback quota reservation
- if error occurred then update instance to ERROR state and rollback quota reservation
- nova.scheduler.filter_scheduler.schedule_prep_resize()
- run through scheduler filters to select host
- call using amqp rpc into nova manager prep_resize
- nova.compute.manager.prep_resize()
- if no node specified call driver.get_available_nodes()
- call _prep_resize
- if an exception occurs then call into scheduler to prep_resize again if possible
- nova.compute.manager._prep_resize()
- if same host is used then ensure that the same host is allowed (as per configuration)
- call using amqp rpc into nova manager resize_instance
- nova.compute.manager.resize_instance()
- get network and instance information
- update instance to RESIZE_MIGRATING state
- get block device information
- call driver.migrate_disk_and_power_off()
- call _terminate_volume_connections
- call into conductor to network_migrate_instance_start which will eventually call the network API migrate_instance_start
- update instance to RESIZE_MIGRATED state
- call using amqp rpc into nova manager finish_resize
- nova.compute.manager._terminate_volume_connections()
- if there is a volume connection to terminate
- driver.get_volume_connector()
- for each volume connection remove the connection by calling the volume API terminate_connection
- if there is a volume connection to terminate
- nova.compute.manager.finish_resize()
- call _finish_resize
- if successful commit the quota reservation
- else rollback the quota reservation and update instance to ERROR state
- nova.compute.manager._finish_resize()
- if the flavor is changing then update the instance with the new flavor
- setup networks on destination host by calling the network API setup_networks_on_host
- call into conductor to network_migrate_instance_finish which will eventually call the network API migrate_instance_finish
- update instance to RESIZE_FINISHED state
- refresh and get block device information
- driver.finish_migration()
- update instance to RESIZED state
Cold migration confirm flow:
- nova.api.openstack.compute.servers._action_confirm_resize()
- nova.compute.api.confirm_resize()
- reserve quota for decrease in resource usage
- call amqp rpc into nova manager confirm_resize
- nova.compute.manager.confirm_resize()
- tear down networks on source host by calling the network API setup_networks_on_host
- driver.confirm_migration()
- update instance to ACTIVE (or possibly STOPPED) state
- commit the quota reservation
Cold migration revert flow:
- nova.api.openstack.compute.servers._action_revert_resize()
- nova.compute.api.revert_resize()
- reserve quota for increase in resource usage
- update instance task state to RESIZE_REVERTING
- call amqp rpc into nova manager revert_resize
- nova.compute.manager.revert_resize()
- tear down networks on destination host by calling the network API setup_networks_on_host
- call into conductor to network_migrate_instance_start which will eventually call the network API migrate_instance_start
- get block device information
- driver.destroy()
- call _terminate_volume_connections
- drop resize resources claimed on destination
- call amqp rpc into nova manager finish_revert_resize
- nova.compute.manager.finish_revert_resize()
- update instance back to pre-resize values
- re-setup networks on source host by calling the network API setup_networks_on_host
- refresh and get block device information
- driver.finish_revert_migration()
- update instance to RESIZE_REVERTING state
- call into conductor to network_migrate_instance_finish which will eventually call the network API migrate_instance_finish
- update instance to ACTIVE (or possibly STOPPED) state
- commit the quota usage