def create_image(self, prepare_template, base, size, *args, **kwargs):
# 调用 'rbd', 'import' 命令将 image file 的数据保存到 rbd image 中

import [–image-format format-id] [–order bits] [–stripe-unit size-in-B/K/M –stripe-count num] [–image-feature feature-name]... [–image-shared] src-path[image-spec]
Creates a new image and imports its data from path (use - for stdin). The import operation will try to create sparse rbd images if possible. For import from stdin, the sparsification unit is the data block size of the destination image (1 << order).
The –stripe-unit and –stripe-count arguments are optional, but must be used together.

root@ceph1:~# rbd ls vms
74cbdb41-3789-4eae-b22e-5085de8caba8_disk.local

root@ceph1:~# rbd info vms/74cbdb41-3789-4eae-b22e-5085de8caba8_disk.local
rbd image '74cbdb41-3789-4eae-b22e-5085de8caba8_disk.local':
size 1024 MB in 256 objects
order 22 (4096 kB objects)
block_name_prefix: rbd_data.11552ae8944a
format: 2
features: layering

<devices>
    <disk type="network" device="disk">
      <driver type="raw" cache="writeback" discard="unmap"/>
      <source protocol="rbd" name="vms/74cbdb41-3789-4eae-b22e-5085de8caba8_disk.local">
        <host name="9.115.251.194" port="6789"/>
        <host name="9.115.251.195" port="6789"/>
        <host name="9.115.251.218" port="6789"/>
      </source>
      <auth username="cinder">
        <secret type="ceph" uuid="e21a123a-31f8-425a-86db-7204c33a6161"/>
      </auth>
      <target bus="virtio" dev="vdb"/>
    </disk>
    <disk type="network" device="disk">
      <driver name="qemu" type="raw" cache="writeback"/>
      <source protocol="rbd" name="volumes/volume-6868a043-1412-4f6c-917f-bbffb1a8d21a">
        <host name="9.115.251.194" port="6789"/>
        <host name="9.115.251.195" port="6789"/>
        <host name="9.115.251.218" port="6789"/>
      </source>
      <auth username="cinder">
        <secret type="ceph" uuid="e21a123a-31f8-425a-86db-7204c33a6161"/>
      </auth>
      <target bus="virtio" dev="vda"/>
      <serial>6868a043-1412-4f6c-917f-bbffb1a8d21a</serial>
    </disk>

def clone(self, context, image_id_or_uri):
1. 通过 Glance API 获取 image 的 rbd location
2. 检查 rbd image 是否可以被克隆（检查它是不是在本 ceph cluster 内、是不是 raw 格式、是不是可以访问等）
3. 调用 rbd 的 clone 方法来创建 clone

def _create_image(self, context, instance,
                      disk_mapping, suffix='',
                      disk_images=None, network_info=None,
                      block_device_info=None, files=None,
                      admin_pass=None, inject_files=True,
                      fallback_from_host=None):
 
      if not booted_from_volume: #如果不是从 volume 启动虚机
            root_fname = imagecache.get_cache_fname(disk_images, 'image_id')
            size = instance.root_gb * units.Gi
            backend = image('disk')
            if backend.SUPPORTS_CLONE: #如果 image backend 支持 clone 的话（目前的各种 image backend，只有 RBD 支持 clone）
                def clone_fallback_to_fetch(*args, **kwargs):
                        backend.clone(context, disk_images['image_id']) #直接调用 backend 的 clone 函数做 image clone
                fetch_func = clone_fallback_to_fetch
            else:
                fetch_func = libvirt_utils.fetch_image #否则走常规的 image 下载-导入过程
            self._try_fetch_image_cache(backend, fetch_func, context, root_fname, disk_images['image_id'], instance, size, fallback_from_host)