diff --git a/server/src/main/java/io/druid/segment/realtime/plumber/RealtimePlumber.java b/server/src/main/java/io/druid/segment/realtime/plumber/RealtimePlumber.java
index 80bd59235c5..e9b0ca388a2 100644
--- a/server/src/main/java/io/druid/segment/realtime/plumber/RealtimePlumber.java
+++ b/server/src/main/java/io/druid/segment/realtime/plumber/RealtimePlumber.java
@@ -68,7 +68,6 @@ import java.util.concurrent.ScheduledExecutorService;
 public class RealtimePlumber implements Plumber
 {
   private static final EmittingLogger log = new EmittingLogger(RealtimePlumber.class);
-
   private final Period windowPeriod;
   private final File basePersistDirectory;
   private final IndexGranularity segmentGranularity;
@@ -84,16 +83,15 @@ public class RealtimePlumber implements Plumber
   private final SegmentPublisher segmentPublisher;
   private final ServerView serverView;
   private final int maxPendingPersists;
-
   private final Object handoffCondition = new Object();
   private final Map<Long, Sink> sinks = Maps.newConcurrentMap();
   private final VersionedIntervalTimeline<String, Sink> sinkTimeline = new VersionedIntervalTimeline<String, Sink>(
       String.CASE_INSENSITIVE_ORDER
   );
-
   private volatile boolean shuttingDown = false;
   private volatile boolean stopped = false;
   private volatile ExecutorService persistExecutor = null;
+  private volatile ExecutorService mergeExecutor = null;
   private volatile ScheduledExecutorService scheduledExecutor = null;
 
   public RealtimePlumber(
@@ -306,7 +304,7 @@ public class RealtimePlumber implements Plumber
     final String threadName = String.format(
         "%s-%s-persist-n-merge", schema.getDataSource(), new DateTime(truncatedTime)
     );
-    persistExecutor.execute(
+    mergeExecutor.execute(
         new ThreadRenamingRunnable(threadName)
         {
           @Override
@@ -315,10 +313,10 @@ public class RealtimePlumber implements Plumber
             final Interval interval = sink.getInterval();
 
             for (FireHydrant hydrant : sink) {
-              if (!hydrant.hasSwapped()) {
-                log.info("Hydrant[%s] hasn't swapped yet, swapping. Sink[%s]", hydrant, sink);
-                final int rowCount = persistHydrant(hydrant, schema, interval);
-                metrics.incrementRowOutputCount(rowCount);
+                if (!hydrant.hasSwapped()) {
+                  log.info("Hydrant[%s] hasn't swapped yet, swapping. Sink[%s]", hydrant, sink);
+                  final int rowCount = persistHydrant(hydrant, schema, interval);
+                  metrics.incrementRowOutputCount(rowCount);
               }
             }
 
@@ -431,6 +429,13 @@ public class RealtimePlumber implements Plumber
           "plumber_persist_%d", maxPendingPersists
       );
     }
+    if (persistExecutor == null) {
+      // use a blocking single threaded executor to throttle the firehose when write to disk is slow
+      mergeExecutor = Execs.newBlockingSingleThreaded(
+          "plumber_persist_%d", maxPendingPersists
+      );
+    }
+
     if (scheduledExecutor == null) {
       scheduledExecutor = Executors.newScheduledThreadPool(
           1,
@@ -592,7 +597,11 @@ public class RealtimePlumber implements Plumber
                     log.info("Adding entry[%s] for merge and push.", entry);
                     sinksToPush.add(entry);
                   } else {
-                    log.warn("[%s] < [%s] Skipping persist and merge.", new DateTime(intervalStart), minTimestampAsDate);
+                    log.warn(
+                        "[%s] < [%s] Skipping persist and merge.",
+                        new DateTime(intervalStart),
+                        minTimestampAsDate
+                    );
                   }
                 }
 
@@ -660,39 +669,46 @@ public class RealtimePlumber implements Plumber
    */
   protected int persistHydrant(FireHydrant indexToPersist, Schema schema, Interval interval)
   {
-    if (indexToPersist.hasSwapped()) {
+    synchronized (indexToPersist) {
+      if (indexToPersist.hasSwapped()) {
+        log.info(
+            "DataSource[%s], Interval[%s], Hydrant[%s] already swapped. Ignoring request to persist.",
+            schema.getDataSource(), interval, indexToPersist
+        );
+        return 0;
+      }
+
       log.info(
-          "DataSource[%s], Interval[%s], Hydrant[%s] already swapped. Ignoring request to persist.",
-          schema.getDataSource(), interval, indexToPersist
+          "DataSource[%s], Interval[%s], persisting Hydrant[%s]",
+          schema.getDataSource(),
+          interval,
+          indexToPersist
       );
-      return 0;
-    }
+      try {
+        int numRows = indexToPersist.getIndex().size();
 
-    log.info("DataSource[%s], Interval[%s], persisting Hydrant[%s]", schema.getDataSource(), interval, indexToPersist);
-    try {
-      int numRows = indexToPersist.getIndex().size();
+        File persistedFile = IndexMerger.persist(
+            indexToPersist.getIndex(),
+            new File(computePersistDir(schema, interval), String.valueOf(indexToPersist.getCount()))
+        );
 
-      File persistedFile = IndexMerger.persist(
-          indexToPersist.getIndex(),
-          new File(computePersistDir(schema, interval), String.valueOf(indexToPersist.getCount()))
-      );
+        indexToPersist.swapSegment(
+            new QueryableIndexSegment(
+                indexToPersist.getSegment().getIdentifier(),
+                IndexIO.loadIndex(persistedFile)
+            )
+        );
 
-      indexToPersist.swapSegment(
-          new QueryableIndexSegment(
-              indexToPersist.getSegment().getIdentifier(),
-              IndexIO.loadIndex(persistedFile)
-          )
-      );
+        return numRows;
+      }
+      catch (IOException e) {
+        log.makeAlert("dataSource[%s] -- incremental persist failed", schema.getDataSource())
+           .addData("interval", interval)
+           .addData("count", indexToPersist.getCount())
+           .emit();
 
-      return numRows;
-    }
-    catch (IOException e) {
-      log.makeAlert("dataSource[%s] -- incremental persist failed", schema.getDataSource())
-         .addData("interval", interval)
-         .addData("count", indexToPersist.getCount())
-         .emit();
-
-      throw Throwables.propagate(e);
+        throw Throwables.propagate(e);
+      }
     }
   }