The error log is like:
2015-07-21 11:20:27,082 DEBUG [apibinding.api] async call[url: http://10.0.101.20:8080/zstack/api/, response: {"org.zstack.header.image.APICreateRootVolumeTemplateFromVolumeSnapshotEvent":{"success":false,"error":{"code":"SYS.1006","description":"An operation failed","details":"failed to rebase and merge snapshots for template[uuid:e447afaa89644caba319d98ac99981bc] on nfs primary storage[uuid:9809a7dbdb0d4a72836d18668e49429e], cannot find mount path of primary storage[uuid: None]"}}}]

The reproduce step:

1. create volume and create vm in NFS PS.
2. attach volume and detach
3. create volume snapshot
4. backup volume snapshot
5. destroy VM.
6. create volume template from backuped volume snapshot.