1、在/var/lib/nova/instances/_base下的这些文件其实是OpenStack通过如下命令得到的，他们是基础镜像。所以目录名为：_base

# qemu-img convert -f qcow2 -O raw /var/lib/glance/images/0da54412-644c-4e59-88b3-2cc16b78ede4 /var/lib/nova/instances/_base/a80e8d64aae8ce6c240588992ab4669a3e64c464


2、而/var/lib/nova/instances/16bcf028-9eea-45b6-ba9d-c99733d50008/disk，它只是基于基础镜像的增量部分。通过如下命令你就能更加清楚其中的意思了

# qemu-img info /var/lib/nova/instances/16bcf028-9eea-45b6-ba9d-c99733d50008/disk
image: /var/lib/nova/instances/16bcf028-9eea-45b6-ba9d-c99733d50008/disk
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 80M
cluster_size: 65536
backing file: /var/lib/nova/instances/_base/a80e8d64aae8ce6c240588992ab4669a3e64c464
backing file format: raw
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false


3、可以通过如下命令修改镜像文件的 backing_file

# qemu-img rebase -b <backing_file> <image_file>