I'm trying to upgrade my elasticsearch cluster from 6.1.1 to 6.1.2 and even though there are no pending_tasks anymore (after I switched that node to 6.1.2):
# curl --silent --request GET $ELASTICSEARCH_URI/_cluster/allocation/explain?pretty | grep explanation | tail -1
"explanation" : "target node version [6.1.1] is older than the source node version [6.1.2] and may not support codecs or postings formats for a newer Lucene version"
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