Warnings of "IllegalStateException: unexpected docvalues type NONE for field '_parent'" on upgraded index

[abhiCR7]

It is the same problem that was reported with this issue.
My current version 5.5.2, build date 14th Aug 2017.
Exceptions I'm getting are:
[2017-08-23T18:05:35,501][WARN ][o.e.i.w.ShardIndexWarmerService] [node_1] [index1][0] failed to warm-up global ordinals for [_parent] org.elasticsearch.ElasticsearchException: java.util.concurrent.ExecutionException: java.lang.IllegalStateException: unexpected docvalues type NONE for field '_parent' (expected one of [SORTED, SORTED_SET]). Re-index with correct docvalues type. at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.loadGlobal(SortedSetDVOrdinalsIndexFieldData.java:120) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.loadGlobal(SortedSetDVOrdinalsIndexFieldData.java:45) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.IndexWarmer$FieldDataWarmer.lambda$warmReader$1(IndexWarmer.java:141) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:569) [elasticsearch-5.5.2.jar:5.5.2] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_91] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_91] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_91] Caused by: java.util.concurrent.ExecutionException: java.lang.IllegalStateException: unexpected docvalues type NONE for field '_parent' (expected one of [SORTED, SORTED_SET]). Re-index with correct docvalues type. at org.elasticsearch.common.cache.Cache.computeIfAbsent(Cache.java:404) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.indices.fielddata.cache.IndicesFieldDataCache$IndexFieldCache.load(IndicesFieldDataCache.java:154) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.loadGlobal(SortedSetDVOrdinalsIndexFieldData.java:115) ~[elasticsearch-5.5.2.jar:5.5.2] ... 6 more Caused by: java.lang.IllegalStateException: unexpected docvalues type NONE for field '_parent' (expected one of [SORTED, SORTED_SET]). Re-index with correct docvalues type. at org.apache.lucene.index.DocValues.checkField(DocValues.java:212) ~[lucene-core-6.6.0.jar:6.6.0 5c7a7b65d2aa7ce5ec96458315c661a18b320241 - ishan - 2017-05-30 07:29:46] at org.apache.lucene.index.DocValues.getSortedSet(DocValues.java:306) ~[lucene-core-6.6.0.jar:6.6.0 5c7a7b65d2aa7ce5ec96458315c661a18b320241 - ishan - 2017-05-30 07:29:46] at org.elasticsearch.index.fielddata.plain.SortedSetDVBytesAtomicFieldData.getOrdinalsValues(SortedSetDVBytesAtomicFieldData.java:53) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.ordinals.GlobalOrdinalsBuilder.build(GlobalOrdinalsBuilder.java:63) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.localGlobalDirect(SortedSetDVOrdinalsIndexFieldData.java:127) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.localGlobalDirect(SortedSetDVOrdinalsIndexFieldData.java:45) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.indices.fielddata.cache.IndicesFieldDataCache$IndexFieldCache.lambda$load$1(IndicesFieldDataCache.java:157) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.common.cache.Cache.computeIfAbsent(Cache.java:401) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.indices.fielddata.cache.IndicesFieldDataCache$IndexFieldCache.load(IndicesFieldDataCache.java:154) ~[elasticsearch-5.5.2.jar:5.5.2] at org.elasticsearch.index.fielddata.plain.SortedSetDVOrdinalsIndexFieldData.loadGlobal(SortedSetDVOrdinalsIndexFieldData.java:115) ~[elasticsearch-5.5.2.jar:5.5.2] ... 6 more

It would be really helpful if anyone can help me with this. The migration-helper plugin suggested no action required.

[jimczi]

As mentioned in the fix this should not be a problem. We introduced this bug in 5.5 but it doesn't mean that your index or cluster is broken. It's annoying because the exception is logged on every refresh (and at startup) but it's only because it tries to load global ordinals for a field that does not exist. There is a fix in the upcoming release of es (5.6), in the mean time you can just ignore the exception since it doesn't prevent the use of the parent/child feature.

[abhiCR7]

@jimczi Thanks for the clarification. Though seeing the warn i was relieved but wanted to be sure before I go ahead with the upgrade in production from 2.x to 5.x