openstack云主机快照

云主机快照功能实现：
OpenStack虚拟机启动源支持镜像和卷，这两种方式创建出的虚拟机，快照过程
也不同。

1：由镜像启动的虚拟机做快照时，只对系统盘进行快照，不对数据盘进行快照，快照过程
中要挂起虚拟机，因此虚拟机中运行的业务可能会中断。快照完成后，将快照上传到
glance服务中，保存为snapshot类型的镜像，并恢复虚拟机状态。
2：由卷启动的虚拟机做快照时，同时对系统盘和数据盘进行快照，快照实际执行由cinder
存储服务完成。因此可以利用存储后端的快照功能。当cinder存储后端为ceph时，就可以
利用ceph的增量快照功能，降低快照时间，减少快照过程对于虚拟机中运行业务的影响。
具体流程如下：

boot from volume vs boot from image in nova snapshot

* boot from volume* nova snapshot will create snapshot for all attached cinder volume
* boot from image* nova snapshot will not create snapshot for any attached cinder volume* 

从 api 层，对于两种启动方式就做了不同的处理，具体看 [ def _action_create_image(self, req, id, body):](https://github.com/openstack/nova/blob/6c52612fb5d25b3e23a7b4d0b8c1f91a3d9b4b41/nova/api/openstack/compute/servers.py#L1231-L1246)```python
try:if compute_utils.is_volume_backed_instance(context, instance,bdms):context.can(server_policies.SERVERS %'create_image:allow_volume_backed', target=target)image = self.compute_api.snapshot_volume_backed(context,instance,image_name,extra_properties=metadata)else:image = self.compute_api.snapshot(context,instance,image_name,extra_properties=metadata)

对于 boot from volume 的虚拟机，也会生成一个 0 字节的 image snapshot， 他主要拿着volume 的相关信息，当恢复的时候，可以从这个 image snapshot 恢复

[{"guest_format": null,"boot_index": 0,"delete_on_termination": true,"no_device": null,"snapshot_id": "1e9b1bbc-13c8-46de-b692-0a5cb6485cd0","volume_type": null,"device_name": "/dev/sda","disk_bus": "scsi","image_id": null,"source_type": "snapshot","tag": null,"device_type": "disk","volume_id": null,"destination_type": "volume","volume_size": 40},{"guest_format": null,"boot_index": null,"delete_on_termination": false,"no_device": null,"snapshot_id": "5a1548d7-f105-4fbd-afa4-f61a68cdef3b","volume_type": null,"device_name": "/dev/sda","disk_bus": "scsi","image_id": null,"source_type": "snapshot","tag": null,"device_type": "disk","volume_id": null,"destination_type": "volume","volume_size": 40},

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