迁移学习心得，从OpenStack到QEMU

来源：互联网发布：唯品会美工的岗位职责编辑：程序博客网时间：2024/04/29 05:58

最近一直在看有关OpenStack里迁移方面的情况，也顺便往下看了下QEMU里关于migration的实现。在这里分享一下自己学习的体会。

我看的OpenStack为g版本，不久将至的h版本在migration方面应该没有太大的改变。（至少没有从f到g来的大

）

OpenStack关于migration可以分为两类，migration和live migration，migration进入dashboard的管理员模式就使用了，live migration因为dashboard还没加进这个功能，所以需要敲命令行实现(nova live-migration instance_ID)。

migration和live migration无论在功能上还是实现上有很大的不同。migration可以说完全是openstack一手操控的，而live migration实现大部分依赖了libvirt以及qemu。

我们先来看看migration的代码：

\nova\compute\manager.py

@exception.wrap_exception(notifier=notifier, publisher_id=publisher_id())    @reverts_task_state    @wrap_instance_event    @wrap_instance_fault    def resize_instance(self, context, instance, image,                        reservations=None, migration=None, migration_id=None,                        instance_type=None):        """Starts the migration of a running instance to another host."""        if not migration:            migration = self.conductor_api.migration_get(context, migration_id)        with self._error_out_instance_on_exception(context, instance['uuid'],                                                   reservations):            if not instance_type:                instance_type = self.conductor_api.instance_type_get(context,                        migration['new_instance_type_id'])            network_info = self._get_instance_nw_info(context, instance)            migration = self.conductor_api.migration_update(context,                    migration, 'migrating')            instance = self._instance_update(context, instance['uuid'],                    task_state=task_states.RESIZE_MIGRATING,                    expected_task_state=task_states.RESIZE_PREP)            self._notify_about_instance_usage(                context, instance, "resize.start", network_info=network_info)            block_device_info = self._get_instance_volume_block_device_info(                                context, instance)            disk_info = self.driver.migrate_disk_and_power_off(                    context, instance, migration['dest_host'],                    instance_type, network_info,                    block_device_info)            self._terminate_volume_connections(context, instance)            self.conductor_api.network_migrate_instance_start(context,                                                              instance,                                                              migration)            migration = self.conductor_api.migration_update(context,                    migration, 'post-migrating')            instance = self._instance_update(context, instance['uuid'],                    host=migration['dest_compute'],                    node=migration['dest_node'],                    task_state=task_states.RESIZE_MIGRATED,                    expected_task_state=task_states.                    RESIZE_MIGRATING)            self.compute_rpcapi.finish_resize(context, instance,                    migration, image, disk_info,                    migration['dest_compute'], reservations)            self._notify_about_instance_usage(context, instance, "resize.end",                                              network_info=network_info)

非常明显，OpenStack中是通过resize这个命令来实现migration的，那很容易想到migration的实现过程和resize就非常相近了：先shutdown，然后(换个flavor)换个host，接着配置个网络，更新个信息，最后重新启动instance。

然后我们看看OpenStack怎么通过libvirt实现migration的。

\nova\virt\libvirt\driver.py

def migrate_disk_and_power_off(self, context, instance, dest,                                   instance_type, network_info,                                   block_device_info=None):        LOG.debug(_("Starting migrate_disk_and_power_off"),                   instance=instance)        disk_info_text = self.get_instance_disk_info(instance['name'],                block_device_info=block_device_info)        disk_info = jsonutils.loads(disk_info_text)        # copy disks to destination        # rename instance dir to +_resize at first for using        # shared storage for instance dir (eg. NFS).        inst_base = libvirt_utils.get_instance_path(instance)        inst_base_resize = inst_base + "_resize"        shared_storage = self._is_storage_shared_with(dest, inst_base)        # try to create the directory on the remote compute node        # if this fails we pass the exception up the stack so we can catch        # failures here earlier        if not shared_storage:            utils.execute('ssh', dest, 'mkdir', '-p', inst_base)        self.power_off(instance)        block_device_mapping = driver.block_device_info_get_mapping(            block_device_info)        for vol in block_device_mapping:            connection_info = vol['connection_info']            disk_dev = vol['mount_device'].rpartition("/")[2]            self.volume_driver_method('disconnect_volume',                                      connection_info,                                      disk_dev)        try:            utils.execute('mv', inst_base, inst_base_resize)            # if we are migrating the instance with shared storage then            # create the directory.  If it is a remote node the directory            # has already been created            if shared_storage:                dest = None                utils.execute('mkdir', '-p', inst_base)            for info in disk_info:                # assume inst_base == dirname(info['path'])                img_path = info['path']                fname = os.path.basename(img_path)                from_path = os.path.join(inst_base_resize, fname)                if info['type'] == 'qcow2' and info['backing_file']:                    tmp_path = from_path + "_rbase"                    # merge backing file                    utils.execute('qemu-img', 'convert', '-f', 'qcow2',                                  '-O', 'qcow2', from_path, tmp_path)                    if shared_storage:                        utils.execute('mv', tmp_path, img_path)                    else:                        libvirt_utils.copy_image(tmp_path, img_path, host=dest)                        utils.execute('rm', '-f', tmp_path)                else:  # raw or qcow2 with no backing file                    libvirt_utils.copy_image(from_path, img_path, host=dest)        except Exception:            with excutils.save_and_reraise_exception():                self._cleanup_remote_migration(dest, inst_base,                                               inst_base_resize,                                               shared_storage)        return disk_info_text

可以看到migration的操作就是poweroff该instance，然后建立ssh链接，将image从源主机拷贝到目的主机。是否为共享存储的区别不过在于是否真的将image“拷贝”还是仅仅改变一个地址。

所以migration在openstack里面的操作还是简单的，相比之下live migration操作要复杂得多。

live migration很大的一个变化是g版本把这个操作从nova-compute挪到了nova-conductor里了。废话不多说，上代码。

\nova\compute\api.py

@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 = self.update(context, instance,  task_state=task_states.MIGRATING,  expected_task_state=None)self.compute_task_api.migrate_server(context, instance,scheduler_hint={'host': host_name},live=True, rebuild=False, flavor=None,block_migration=block_migration,disk_over_commit=disk_over_commit)

这里的compute_task_api就是指conductor，我们接着看conductor。

\nova\conductor\manager.py

def migrate_server(self, context, instance, scheduler_hint, live, rebuild, flavor, block_migration, disk_over_commit):if not live or rebuild or (flavor != None):raise NotImplementedError()
destination = scheduler_hint.get("host")
self.scheduler_rpcapi.live_migration(context, block_migration,
disk_over_commit, instance, destination)

好吧，conductor把皮球踢给了scheduler，在这里限于篇幅，我们简单看一下scheduler里面做的是什么。

try:self._schedule_live_migration(context, instance, dest,block_migration, disk_over_commit)except (exception.NoValidHost,exception.ComputeServiceUnavailable,exception.InvalidHypervisorType,exception.UnableToMigrateToSelf,exception.DestinationHypervisorTooOld,exception.InvalidLocalStorage,exception.InvalidSharedStorage,exception.MigrationPreCheckError) as ex:request_spec = {'instance_properties': {'uuid': instance['uuid'], },}

scheduler无非检查一下destination的情况，storage的情况等，然后接着把皮球踢还conductor。

\conductor\tasks\live_migrate.py

class LiveMigrationTask(object):def __init__(self, context, instance, destination,block_migration, disk_over_commit,select_hosts_callback):self.context = contextself.instance = instanceself.destination = destinationself.block_migration = block_migrationself.disk_over_commit = disk_over_commitself.select_hosts_callback = select_hosts_callbackself.source = instance['host']self.migrate_data = Noneself.compute_rpcapi = compute_rpcapi.ComputeAPI()self.servicegroup_api = servicegroup.API()self.image_service = glance.get_default_image_service()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 managerreturn 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)

终于干正事了，conductor通过MQ把live migration的消息告诉了源主机的nova-compute。咱们开始迁移吧！老规矩，还是让libvirt来干活～

\nova\virt\libvirt\driver.py

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.:params context: security context:params instance:nova.db.sqlalchemy.models.Instance objectinstance object that is migrated.:params dest: destination host:params block_migration: destination host:params post_method:post operation method.expected nova.compute.manager.post_live_migration.:params recover_method:recovery method when any exception occurs.expected nova.compute.manager.recover_live_migration.:params block_migration: if true, do block migration.:params migrate_data: implementation specific params"""greenthread.spawn(self._live_migration, context, instance, dest, post_method, recover_method, block_migration, migrate_data)def _live_migration(self, context, instance, dest, post_method,recover_method, block_migration=False,migrate_data=None):"""Do live migration.:params context: security context:params instance:nova.db.sqlalchemy.models.Instance objectinstance object that is migrated.:params dest: destination host:params post_method:post operation method.expected nova.compute.manager.post_live_migration.:params recover_method:recovery method when any exception occurs.expected nova.compute.manager.recover_live_migration.:params migrate_data: implementation specific params"""# Do live migration.try:if block_migration:flaglist = CONF.block_migration_flag.split(',')else:flaglist = CONF.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.live_migration_uri % dest,logical_sum,None,CONF.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)

接着就要调用libvirt以及他下面的QEMU了，也就是dom.migrateToURI()这个函数。

如果玩过类似virt manager的东西，对于这个命令里的参数应该不会陌生。事实上，OpenStack里面的live-migration和在virt manager里做migration没什么本质的区别。

到这里总结一下，OpenStack首先在qemu上面找了个libvirt这样的函数库，封装他的API。毕竟OpenStack除了KVM外，还要支持Xen，ESX这样的虚拟化工具。然后相比于单机上的虚拟化，作为云平台的管理工具，OpenStack当然要负责完成把instance的信息在数据库里更新（也就是conductor的任务）、搭建起网络、选择一个目的主机等一系列工序。至于是不是shared storage，其实OpenStack不会太关注，因为libvirt或者更准确的说，QEMU完全能自己搞定这件事了。OpenStack通知他一声就成～

另外，值得一提的是，OpenStack默认是不支持live-migration的，因为OpenStack认为live-migration会因为内存中dirty page rate大于网络的brandwidth而不能停止（详细解释请关注之后QEMU的部分）。因此如果不进行相关设置的话，OpenStack默认会用migration代替（live-migration是不是有种需要打开隐藏关卡才能看到的赶脚）。

总体感觉OpenStack在live migration方面做得还是挺不错的，我在实践中也没发现什么问题。毕竟OpenStack也就是个管理工具，live migration活怎么样（downtime多少，总迁移时间多少，性能下降多少等），和他也没什么直接关系。

反正写到这里OpenStack方面关于迁移的事情我觉得交代的差不多了，接着讲讲QEMU里是怎么实现的吧。（未完待续）

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