OpenStack 创建快照(Create Snapshot)过程源码分析

1. 背景：
    一直以为OpenStack的创建快照的操作是在线创建快照（live snapshot）, 并且应该是增量的快照，即利用virsh或者qemu的live snapshot来实现的：
         virsh  snapshot-create-as --live ....
    后来发现快照和原始镜像之间并没有依赖关系，感觉OpenStack还做的挺好的，自动解决了增量快照和原始镜像之间的依赖关系;
    但是后来又发现做快照的时候虚拟机竟然会shutoff, 就感觉不对了，于是分析了下源码。

2. 结论：目前OpenStack默认的快照方式都是cold snapshot, 首先先关机，其次执行如下命令生成一个镜像文件，再次开机，最后再调用glance api将镜像上传。
       qemu-img convert -f qcow2 -O qcow2  
       所以目前并不是真正意义的快照，其实和关闭虚拟机，拷贝一份，再上传没有本质区别。

3. 源代码流程分析

3.1 nova/compute/api.py

# NOTE(melwitt): We don't check instance lock for snapshot because lock is
    #                intended to prevent accidental change/delete of instances
    @wrap_check_policy
    @check_instance_cell
    @check_instance_state(vm_state=[vm_states.ACTIVE, vm_states.STOPPED,
                                    vm_states.PAUSED, vm_states.SUSPENDED])
    def snapshot(self, context, instance, name, extra_properties=None):
        """Snapshot the given instance.

        :param instance: nova.db.sqlalchemy.models.Instance
        :param name: name of the snapshot
        :param extra_properties: dict of extra image properties to include
                                 when creating the image.
        :returns: A dict containing image metadata
        """
       #调用glance api创建image entry，为后将snapshot上传为镜像做准备，虽然镜像和snapshot在可以上传到glance作为镜像启动虚拟机，
       #但是为了区分二者的不同，glance将镜像和snapshot标记卫不同的类型：type=image 和 type=snapshot
        image_meta = self._create_image(context, instance, name,
                                        'snapshot',
                                        extra_properties=extra_properties)

        # NOTE(comstud): Any changes to this method should also be made
        # to the snapshot_instance() method in nova/cells/messaging.py
        # 将任务状态(task state) 设置为:image_snapshot_pending
        instance.task_state = task_states.IMAGE_SNAPSHOT_PENDING
        instance.save(expected_task_state=[None])
        #通过rpc调用nova/compute/rpcapi.py的snapshot_instance函数
        self.compute_rpcapi.snapshot_instance(context, instance,
                                              image_meta['id

3.2 nova/compute/rpcapi.py

#梳理下流程： （1）用户发起create snapshot的请求; （2）nova-api服务接收到这个请求并进行前期处理，即3.1中代码的处理流程;
                            （3）真正的snapshot操作是需要在nova-compute节点上执行的，所以nova-api需要向nova-compute发送message
#由于OpenStack环境中会有很多个nova-compute,所以需要通过server=_compute_host(None, instance)来获取虚拟机所在的host，并向其发送message。
def snapshot_instance(self, ctxt, instance, image_id):
        version = '3.0'
        cctxt = self.client.prepare(server=_compute_host(None, instance),
                version=version)
        cctxt.cast(ctxt, 'snapshot_instance',
                   instance=instance,
                   image_id=image_id)

3.3 nova/virt/libvirt/driver.py

 def snapshot(self, context, instance, image_id, update_task_state):
        """Create snapshot from a running VM instance.

        This command only works with qemu 0.14+
        """
        try:
            virt_dom = self._get_domain(instance)
        except exception.InstanceNotFound:
            raise exception.InstanceNotRunning(instance_id=instance['uuid'])

        base_image_ref = instance['image_ref']

        base = compute_utils.get_image_metadata(
            context, self._image_api, base_image_ref, instance)

        snapshot = self._image_api.get(context, image_id)

        disk_path = libvirt_utils.find_disk(virt_dom)
        source_format = libvirt_utils.get_disk_type(disk_path)

        image_format = CONF.libvirt.snapshot_image_format or source_format

        # NOTE(bfilippov): save lvm and rbd as raw
        if image_format == 'lvm' or image_format == 'rbd':
            image_format = 'raw'

        metadata = self._create_snapshot_metadata(base,
                                                  instance,
                                                  image_format,
                                                  snapshot['name'])

        snapshot_name = uuid.uuid4().hex

        state = LIBVIRT_POWER_STATE[virt_dom.info()[0]]

        # NOTE(rmk): Live snapshots require QEMU 1.3 and Libvirt 1.0.0.
        #            These restrictions can be relaxed as other configurations
        #            can be validated.
        # NOTE(dgenin): Instances with LVM encrypted ephemeral storage require
        #               cold snapshots. Currently, checking for encryption is
        #               redundant because LVM supports only cold snapshots.
        #               It is necessary in case this situation changes in the
        #               future.
        #这里需要注意，解释了为啥现在是cold snapshot而不是live snapshot:
        # 有人提过live snapshot的bug，社区认为live snapshot目前不稳定，所以默认条件下采用cold snapshot，并且是通过硬编码来实现的
        # 看下面这个判断条件，成立的时候将live_snapshot = true,其中MIN_LIBVIRT_LIVESNAPSHOT_VERSION=1.3.0, 其实现在libvirt的最新版本
        # 才到1.2.11， 所以这个live_snapshot的条件不满足，就变成了cold_snapshot
        if (self._host.has_min_version(MIN_LIBVIRT_LIVESNAPSHOT_VERSION,
                                       MIN_QEMU_LIVESNAPSHOT_VERSION,
                                       REQ_HYPERVISOR_LIVESNAPSHOT)
             and source_format not in ('lvm', 'rbd')
             and not CONF.ephemeral_storage_encryption.enabled):
            live_snapshot = True
            # Abort is an idempotent operation, so make sure any block
            # jobs which may have failed are ended. This operation also
            # confirms the running instance, as opposed to the system as a
            # whole, has a new enough version of the hypervisor (bug 1193146).
            try:
                virt_dom.blockJobAbort(disk_path, 0)
            except libvirt.libvirtError as ex:
                error_code = ex.get_error_code()
                if error_code == libvirt.VIR_ERR_CONFIG_UNSUPPORTED:
                    live_snapshot = False
                else:
                    pass
        else:
            live_snapshot = False

        # NOTE(rmk): We cannot perform live snapshots when a managedSave
        #            file is present, so we will use the cold/legacy method
        # for instances which are shutdown.
        if state == power_state.SHUTDOWN:
            live_snapshot = False

        # NOTE(dkang): managedSave does not work for LXC
        #注意这里，如果live_snashot目前是false,所以在做snapshot之前先要执行：
        #（1）_detach_pci_devices， 卸载虚拟机挂载的pci设备，比如数据盘
        #（2）  self._detach_sriov_ports， 卸载虚拟机挂载的SRIOV设备，比如支持SRIOV的网卡设备
        if CONF.libvirt.virt_type != 'lxc' and not live_snapshot:
            if state == power_state.RUNNING or state == power_state.PAUSED:
                self._detach_pci_devices(virt_dom,
                    pci_manager.get_instance_pci_devs(instance))
                self._detach_sriov_ports(instance, virt_dom)
                virt_dom.managedSave(0)
        #判断虚拟机的后端存储是什么，不同的后端存储做snapshot是不同的，本地文件系统的化，默认qcow2
        snapshot_backend = self.image_backend.snapshot(instance,
                disk_path,
                image_type=source_format)

        if live_snapshot:
            LOG.info(_LI("Beginning live snapshot process"),
                     instance=instance)
        else:
            LOG.info(_LI("Beginning cold snapshot process"),
                     instance=instance)
        #更新任务的状态为：image_pending_upload, 大家都知道做完snapshot要上传
        update_task_state(task_state=task_states.IMAGE_PENDING_UPLOAD)
        #目前做快照的过程是：
        #（1）现在../data/nova/instance/snapshots目录下生成临时目录，比如nova/instances/snapshots/tmptHr585
        #（2）然后将快照生成到这个目录，具体参见snapshot_backend.snapshot_extract(out_path, image_format)这个函数
        #（3）生成完成后，通过glance api上传，具体参见 self._image_api.update
        snapshot_directory = CONF.libvirt.snapshots_directory
        fileutils.ensure_tree(snapshot_directory)
        with utils.tempdir(dir=snapshot_directory) as tmpdir:
            try:
                out_path = os.path.join(tmpdir, snapshot_name)
                if live_snapshot:
                    # NOTE(xqueralt): libvirt needs o+x in the temp directory
                    os.chmod(tmpdir, 0o701)
                    self._live_snapshot(context, instance, virt_dom, disk_path,
                                        out_path, image_format, base)
                else:
                    #这个函数实际执行了一条命令： qemu-img convert -f qcow2 -O qcow2  disk_path out_path,算是生成了快照
                    snapshot_backend.snapshot_extract(out_path, image_format)
            finally:
                new_dom = None
                # NOTE(dkang): because previous managedSave is not called
                # for LXC, _create_domain must not be called.
                if CONF.libvirt.virt_type != 'lxc' and not live_snapshot:
                    if state == power_state.RUNNING:
                        new_dom = self._create_domain(domain=virt_dom)   ##恢复做快照之前虚拟机的状态
                    elif state == power_state.PAUSED:
                        new_dom = self._create_domain(domain=virt_dom,
                                launch_flags=libvirt.VIR_DOMAIN_START_PAUSED)
                    if new_dom is not None:
                        self._attach_pci_devices(new_dom,
                            pci_manager.get_instance_pci_devs(instance))
                        self._attach_sriov_ports(context, instance, new_dom)
                LOG.info(_LI("Snapshot extracted, beginning image upload"),
                         instance=instance)

            # Upload that image to the image service

            update_task_state(task_state=task_states.IMAGE_UPLOADING,
                     expected_state=task_states.IMAGE_PENDING_UPLOAD)
            with libvirt_utils.file_open(out_path) as image_file:       ###将生成的快照上传到glance
                self._image_api.update(context,
                                       image_id,
                                       metadata,
                                       image_file)
                LOG.info(_LI("Snapshot image upload complete"),
                         instance=instance)

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