Elasticsearch 2.1 _uid is reformated, shard will failed

this form maybe met the same problem:
received shard failed for [api-nginx-logs-2016.01.27][0], node[qIW6bRNhRH-f6c4d4mL0_g], [R], v[5], s[STARTED], a[id=LLmaqrfqRTSJeHVoBGGUKg], indexUUID [PkwVMa-pR96vk30tXQZChA], message [failed to update mappings], failure [IllegalArgumentException[Mapper for [_uid] conflicts with existing mapping in other types:
[mapper [_uid] cannot be changed from type [_uid] to [string]]]]
if is a bug in 2.1?

It's not a bug, it's ES stopping you from having bad mappings.

Can you provide more information, what does the data look like?