【编者的话】
笔者以一个普通用户的角度，在实际应用的过程中不断深入对看OpenStack的理解。在遇到故障时，通过对源码流程的分析，来探求问题出现的原因是知道在操作上还是系统上。本篇博文主要针对作者对创建快照过程（Create Snapshot）的源代码流程的分析，走出了之前的认知误区，理解了目前OpenStack创建快照过程的实质。
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。
源代码流程分析
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
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 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***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)
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，***再调用glance api将镜像上传。
- qemu-img convert -f qcow2 -O qcow2 <disk_path> <out_path>
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。
