OpenStack nova VM migration (live and cold) call flow
OpenStack nova compute supports two flavors of Virtual Machine (VM) migration:[*]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
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
[*]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
Source: http://bodenr.blogspot.com/2014/03/openstack-nova-vm-migration-live-and.html
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