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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