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：
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；
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；
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。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持VEVB武林网。
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