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源代码流程分析 1 nova/compute/api.py # NOTE(melwitt): We dont 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) 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
