ref：
官方文档很给力：
nova 迁移有以下类型：
- Non-live migration (sometimes referred to simply as ‘migration’). The instance is shut down for a period of time to be moved to another hypervisor. In this case, the instance recognizes that it was rebooted.
- Live migration (or ‘true live migration’). Almost no instance downtime. Useful when the instances must be kept running during the migration. The different types of live migration are:
- Shared storage-based live migration. Both hypervisors have access to shared storage.
- Block live migration. No shared storage is required. Incompatible with read-only devices such as CD-ROMs andConfiguration Drive (config_drive).
- Volume-backed live migration. Instances are backed by volumes rather than ephemeral disk, no shared storage is required, and migration is supported (currently only available for libvirt-based hypervisors).
具体配置方法：http://docs.openstack.org/admin-guide/compute-configuring-migrations.html
下边也是一些资料：
《虚拟机在 OpenStack 里没有共享存储条件下的在线迁移》
http://www.ibm.com/developerworks/cn/cloud/library/1508_wangyx_openstacklivemigrate/
《基于Openstack的虚拟机热迁移》
http://www.voidcn.com/blog/just_young/article/p-1929602.html
《配置openstack基于NFS共享存储的虚拟机在线迁移》
http://f.dataguru.cn/thread-614534-1-1.html