Reduce index.shard.recovery.concurrent_streams from 5 to 3 to reduce the load when doing recovery, closes elastic#2199.

the index.shard.recovery.concurrent_streams controls how many streams are opened from a recovery source to a recovery target to transfer index files. Reduce it from 5 to 3 to reduce the load when doing recovery (for example, due to relocation).

Note, recent changes in network buffering will mean that recovery will progress considerably faster, so this change will not affect recovery times.

Author: kimchy

src/main/java/org/elasticsearch/indices/recovery/RecoverySettings.java

@@ -67,7 +67,7 @@ public RecoverySettings(Settings settings, NodeSettingsService nodeSettingsServi
         this.translogSize = componentSettings.getAsBytesSize("translog_size", settings.getAsBytesSize("index.shard.recovery.translog_size", new ByteSizeValue(512, ByteSizeUnit.KB)));
         this.compress = componentSettings.getAsBoolean("compress", true);
 
-        this.concurrentStreams = componentSettings.getAsInt("concurrent_streams", settings.getAsInt("index.shard.recovery.concurrent_streams", 5));
+        this.concurrentStreams = componentSettings.getAsInt("concurrent_streams", settings.getAsInt("index.shard.recovery.concurrent_streams", 3));
         this.concurrentStreamPool = EsExecutors.newScalingExecutorService(0, concurrentStreams, 60, TimeUnit.SECONDS, EsExecutors.daemonThreadFactory(settings, "[recovery_stream]"));
 
         this.maxSizePerSec = componentSettings.getAsBytesSize("max_size_per_sec", new ByteSizeValue(0));