Failed to put mappings on indices timeouts

Hi I'm seeing a lot of put mapping timeouts on my master server and have a few questions related to this. The exact log line is:

Sep 20 16:00:14.533921 ops-flume-es-master-useast1-2-i-ba0a66ac ElasticSearch: [DEBUG][action.admin.indices.mapping.put] [ops-flume-es-master-useast1-2-i-ba0a66ac-flume-elasticsearch-production_vpc-useast1] failed to put mappings on indices [[flume-2016-09-20]], type [log]
Sep 20 16:00:14.534708 ops-flume-es-master-useast1-2-i-ba0a66ac ProcessClusterEventTimeoutException[failed to process cluster event (put-mapping [log]) within 30s]
Sep 20 16:00:14.535102 ops-flume-es-master-useast1-2-i-ba0a66ac at org.elasticsearch.cluster.service.InternalClusterService$2$
Sep 20 16:00:14.535381 ops-flume-es-master-useast1-2-i-ba0a66ac at java.util.concurrent.ThreadPoolExecutor.runWorker(
Sep 20 16:00:14.535752  at java.util.concurrent.ThreadPoolExecutor$
Sep 20 16:00:14.536012 ops-flume-es-master-useast1-2-i-ba0a66ac at

I presume this may be related to pressure. The cluster has 77 indices, 1334 shards, and ~27B docs. We're running ES 2.4.0 and jdk1.8.

A few questions:

  • Is the timeout related to the master it self timing out or nodes not responding to the master in time?
  • Depending on the above what would help? Adding additional nodes to spread the load or scaling up the master(s)?