Zhonghua Jiang : editing a comment with substantial new information, after it has been replied to, breaks the flow of the conversation.
In your example mdadm.conf, you need to replace the ',' before "spare-group=global" with a space.
Also there is a bug in mdadm-2.6.2 which stops spare migration from working. It is fixed in 2.6.3, and not present before 2.6 (I think).