OpenStack之虚机热迁移代码解析

OpenStack之虚机热迁移代码解析

　　话说虚机迁移分为冷迁移以及热迁移，所谓热迁移用度娘的话说即是：热迁移（Live Migration，又叫动态迁移、实时迁移），即虚机保存/恢复(Save/Restore)：将整个虚拟机的运行状态完整保存下来，同时可以快速的恢复到原有硬件平台甚至是不同硬件平台上。恢复以后，虚机仍旧平滑运行，用户不会察觉到任何差异。OpenStack的虚机迁移是基于Libvirt实现的，下面来看看Openstack虚机热迁移的具体代码实现。

　　首先，由API入口进入到nova/api/openstack/compute/contrib/admin_actions.py

  @wsgi.action('os-migrateLive')
     def _migrate_live(self, req, id, body):
         """Permit admins to (live) migrate a server to a new host."""
         context = req.environ["nova.context"]
         authorize(context, 'migrateLive')

         try:
             block_migration = body["os-migrateLive"]["block_migration"]
             disk_over_commit = body["os-migrateLive"]["disk_over_commit"]
             host = body["os-migrateLive"]["host"]
         except (TypeError, KeyError):
             msg = _("host, block_migration and disk_over_commit must "
                     "be specified for live migration.")
             raise exc.HTTPBadRequest(explanation=msg)

         try:
             block_migration = strutils.bool_from_string(block_migration,
                                                         strict=True)
             disk_over_commit = strutils.bool_from_string(disk_over_commit,
                                                          strict=True)
         except ValueError as err:
             raise exc.HTTPBadRequest(explanation=str(err))

         try:
             instance = self.compute_api.get(context, id, want_objects=True)
             self.compute_api.live_migrate(context, instance, block_migration,
                                           disk_over_commit, host)
         except (exception.ComputeServiceUnavailable,
                 exception.InvalidHypervisorType,
                 exception.UnableToMigrateToSelf,
                 exception.DestinationHypervisorTooOld,
                 exception.NoValidHost,
                 exception.InvalidLocalStorage,
                 exception.InvalidSharedStorage,
                 exception.MigrationPreCheckError) as ex:
             raise exc.HTTPBadRequest(explanation=ex.format_message())
         except exception.InstanceNotFound as e:
             raise exc.HTTPNotFound(explanation=e.format_message())
         except exception.InstanceInvalidState as state_error:
             common.raise_http_conflict_for_instance_invalid_state(state_error,
                     'os-migrateLive')
         except Exception:
             if host is None:
                 msg = _("Live migration of instance %s to another host "
                         "failed") % id
             else:
                 msg = _("Live migration of instance %(id)s to host %(host)s "
                         "failed") % {'id': id, 'host': host}
             LOG.exception(msg)
             # Return messages from scheduler
             raise exc.HTTPBadRequest(explanation=msg)

         return webob.Response(status_int=202)

　　  这里第一行可以看到是与API文档的第二行照应的：

 {
     "os-migrateLive": {
         "host": "0443e9a1254044d8b99f35eace132080",
         "block_migration": false,
         "disk_over_commit": false
     }
 }

　　好了，源码中其实执行迁移工作的就是第26、27行的一条语句：

 self.compute_api.live_migrate(context, instance, block_migration,
                                           disk_over_commit, host)

　　由这句进入到nova/compute/api.py中，源码如下：

     @check_instance_cell
     @check_instance_state(vm_state=[vm_states.ACTIVE])
     def live_migrate(self, context, instance, block_migration,
                      disk_over_commit, host_name):
         """Migrate a server lively to a new host."""
         LOG.debug(_("Going to try to live migrate instance to %s"),
                   host_name or "another host", instance=instance)

         instance.task_state = task_states.MIGRATING
         instance.save(expected_task_state=[None])

         self.compute_task_api.live_migrate_instance(context, instance,
                 host_name, block_migration=block_migration,
                 disk_over_commit=disk_over_commit)

　　第2行是一个装饰器，用于在进入API方法之前，检测虚拟机和/或任务的状态， 如果实例处于错误的状态，将会引发异常；接下来实时迁移虚机到新的主机，并将虚机状态置于“migrating”,然后由12行进入nova/conductor/api.py

 def live_migrate_instance(self, context, instance, host_name,
                               block_migration, disk_over_commit):
         scheduler_hint = {'host': host_name}
         self._manager.migrate_server(
             context, instance, scheduler_hint, True, False, None,
             block_migration, disk_over_commit, None)

　　将主机名存入字典scheduler_hint中，然后调用nova/conductor/manager.py方法migrate_server,

 def migrate_server(self, context, instance, scheduler_hint, live, rebuild,
             flavor, block_migration, disk_over_commit, reservations=None):
         if instance and not isinstance(instance, instance_obj.Instance):
             # NOTE(danms): Until v2 of the RPC API, we need to tolerate
             # old-world instance objects here
             attrs = ['metadata', 'system_metadata', 'info_cache',
                      'security_groups']
             instance = instance_obj.Instance._from_db_object(
                 context, instance_obj.Instance(), instance,
                 expected_attrs=attrs)
         if live and not rebuild and not flavor:
             self._live_migrate(context, instance, scheduler_hint,
                                block_migration, disk_over_commit)
         elif not live and not rebuild and flavor:
             instance_uuid = instance['uuid']
             with compute_utils.EventReporter(context, self.db,
                                          'cold_migrate', instance_uuid):
                 self._cold_migrate(context, instance, flavor,
                                    scheduler_hint['filter_properties'],
                                    reservations)
         else:
             raise NotImplementedError()

　　由于在nova/conductor/api.py中传过来的参数是

 self._manager.migrate_server(
             context, instance, scheduler_hint, True, False, None,
             block_migration, disk_over_commit, None)

　　因此live是True，rebuild是Flase,flavor是None，执行第12、13行代码：

 if live and not rebuild and not flavor:
             self._live_migrate(context, instance, scheduler_hint,
                                block_migration, disk_over_commit) 

　　_live_migrate代码如下：

  def _live_migrate(self, context, instance, scheduler_hint,
                       block_migration, disk_over_commit):
         destination = scheduler_hint.get("host")
         try:
             live_migrate.execute(context, instance, destination,
                              block_migration, disk_over_commit)
         except (exception.NoValidHost,
                 exception.ComputeServiceUnavailable,
                 exception.InvalidHypervisorType,
                 exception.InvalidCPUInfo,
                 exception.UnableToMigrateToSelf,
                 exception.DestinationHypervisorTooOld,
                 exception.InvalidLocalStorage,
                 exception.InvalidSharedStorage,
                 exception.HypervisorUnavailable,
                 exception.MigrationPreCheckError) as ex:
             with excutils.save_and_reraise_exception():
                 #TODO(johngarbutt) - eventually need instance actions here
                 request_spec = {'instance_properties': {
                     'uuid': instance['uuid'], },
                 }
                 scheduler_utils.set_vm_state_and_notify(context,
                         'compute_task', 'migrate_server',
                         dict(vm_state=instance['vm_state'],
                              task_state=None,
                              expected_task_state=task_states.MIGRATING,),
                         ex, request_spec, self.db)
         except Exception as ex:
             LOG.error(_('Migration of instance %(instance_id)s to host'
                        ' %(dest)s unexpectedly failed.'),
                        {'instance_id': instance['uuid'], 'dest': destination},
                        exc_info=True)
             raise exception.MigrationError(reason=ex)

　　首先，第三行中将主机名赋给destination,然后执行迁移，后面的都是异常的捕捉，执行迁移的代码分为两部分，先看第一部分，在nova/conductor/tasks/live_migrate.py的184行左右：

 def execute(context, instance, destination,
             block_migration, disk_over_commit):
     task = LiveMigrationTask(context, instance,
                              destination,
                              block_migration,
                              disk_over_commit)
     #TODO(johngarbutt) create a superclass that contains a safe_execute call
     return task.execute()

　　先创建包含安全执行回调的超类，然后返回如下函数也即执行迁移的第二部分代码，在54行左右：

 def execute(self):
         self._check_instance_is_running()
         self._check_host_is_up(self.source)

         if not self.destination:
             self.destination = self._find_destination()
         else:
             self._check_requested_destination()

         #TODO(johngarbutt) need to move complexity out of compute manager
         return self.compute_rpcapi.live_migration(self.context,
                 host=self.source,
                 instance=self.instance,
                 dest=self.destination,
                 block_migration=self.block_migration,
                 migrate_data=self.migrate_data)
                 #TODO(johngarbutt) disk_over_commit?

　　这里有三部分内容：

1. 如果目前主机不存在，则由调度算法选取一个目标主机，并且进行相关的检测，确保能够进行实时迁移操作；
2. 如果目标主机存在，则直接进行相关的检测操作，确保能够进行实时迁移操作；
3. 执行迁移操作。

　　前两部分不再赘述，直接看第三部分代码，在nova/compute/rpcapi.py中：

 def live_migration(self, ctxt, instance, dest, block_migration, host,
                        migrate_data=None):
         # NOTE(russellb) Havana compat
         version = self._get_compat_version('3.0', '2.0')
         instance_p = jsonutils.to_primitive(instance)
         cctxt = self.client.prepare(server=host, version=version)
         cctxt.cast(ctxt, 'live_migration', instance=instance_p,
                    dest=dest, block_migration=block_migration,
                    migrate_data=migrate_data)

　　热迁移开始执行：

  def live_migration(self, context, instance, dest,
                        post_method, recover_method, block_migration=False,
                        migrate_data=None):
         """Spawning live_migration operation for distributing high-load.

         :param context: security context
         :param instance:
             nova.db.sqlalchemy.models.Instance object
             instance object that is migrated.
         :param dest: destination host
         :param post_method:
             post operation method.
             expected nova.compute.manager.post_live_migration.
         :param recover_method:
             recovery method when any exception occurs.
             expected nova.compute.manager.recover_live_migration.
         :param block_migration: if true, do block migration.
         :param migrate_data: implementation specific params

         """

         greenthread.spawn(self._live_migration, context, instance, dest,
                           post_method, recover_method, block_migration,
                           migrate_data)

　　这个方法中建立一个绿色线程来运行方法_live_migration，来执行实时迁移； 主要是调用libvirt python接口方法virDomainMigrateToURI，来实现从当前主机迁移domain对象到给定的目标主机；

　　spawn：建立一个绿色线程来运行方法“func(*args, **kwargs)”，这里就是来运行方法_live_migration；

　　 _live_migration：执行实时迁移； 主要是调用libvirt python接口方法virDomainMigrateToURI，来实现从当前主机迁移domain对象到给定的目标主机；

　　　接着在绿色线程中调用_live_migration方法：

 def _live_migration(self, context, instance, dest, post_method,
                         recover_method, block_migration=False,
                         migrate_data=None):
         """Do live migration.

         :param context: security context
         :param instance:
             nova.db.sqlalchemy.models.Instance object
             instance object that is migrated.
         :param dest: destination host
         :param post_method:
             post operation method.
             expected nova.compute.manager.post_live_migration.
         :param recover_method:
             recovery method when any exception occurs.
             expected nova.compute.manager.recover_live_migration.
         :param block_migration: if true, do block migration.
         :param migrate_data: implementation specific params
         """

         # Do live migration.
         try:
             if block_migration:
                 flaglist = CONF.libvirt.block_migration_flag.split(',')
             else:
                 flaglist = CONF.libvirt.live_migration_flag.split(',')
             flagvals = [getattr(libvirt, x.strip()) for x in flaglist]
             logical_sum = reduce(lambda x, y: x | y, flagvals)

             dom = self._lookup_by_name(instance["name"])
             dom.migrateToURI(CONF.libvirt.live_migration_uri % dest,
                              logical_sum,
                              None,
                              CONF.libvirt.live_migration_bandwidth)

         except Exception as e:
             with excutils.save_and_reraise_exception():
                 LOG.error(_("Live Migration failure: %s"), e,
                           instance=instance)
                 recover_method(context, instance, dest, block_migration)

         # Waiting for completion of live_migration.
         timer = loopingcall.FixedIntervalLoopingCall(f=None)

 if block_migration:
                 flaglist = CONF.libvirt.block_migration_flag.split(',')

　　这个获取块迁移标志列表，block_migration_flag：这个参数定义了为块迁移设置迁移标志。

  else:
                 flaglist = CONF.libvirt.live_migration_flag.split(',')
             flagvals = [getattr(libvirt, x.strip()) for x in flaglist]
             logical_sum = reduce(lambda x, y: x | y, flagvals)

　　这部分获取实时迁移标志列表，live_migration_flag这个参数定义了实时迁移的迁移标志。

 dom = self._lookup_by_name(instance["name"])

　　根据给定的实例名称检索libvirt域对象。

 timer = loopingcall.FixedIntervalLoopingCall(f=None)

　　获取等待完成实时迁移的时间。

　　热迁移代码部分至此结束。

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