HDFS-7060. Avoid taking locks when sending heartbeats from the DataNode. Contributed by Jiandan Yang.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java

@@ -153,7 +153,8 @@ public DatanodeStorage getStorage(final String storageUuid) {
   public StorageReport[] getStorageReports(String bpid)
       throws IOException {
     List<StorageReport> reports;
-    synchronized (statsLock) {
+    // Volumes are the references from a copy-on-write snapshot, so the
+    // access on the volume metrics doesn't require an additional lock.
     List<FsVolumeImpl> curVolumes = volumes.getVolumes();
     reports = new ArrayList<>(curVolumes.size());
     for (FsVolumeImpl volume : curVolumes) {
@@ -170,7 +171,6 @@ public StorageReport[] getStorageReports(String bpid)
         continue;
       }
     }
-    }
 
     return reports.toArray(new StorageReport[reports.size()]);
   }
@@ -247,9 +247,6 @@ public LengthInputStream getMetaDataInputStream(ExtendedBlock b)
 
   private final int smallBufferSize;
 
-  // Used for synchronizing access to usage stats
-  private final Object statsLock = new Object();
-
   final LocalFileSystem localFS;
 
   private boolean blockPinningEnabled;
@@ -583,20 +580,16 @@ public void removeVolumes(
    */
   @Override // FSDatasetMBean
   public long getDfsUsed() throws IOException {
-    synchronized(statsLock) {
     return volumes.getDfsUsed();
   }
-  }
 
   /**
    * Return the total space used by dfs datanode
    */
   @Override // FSDatasetMBean
   public long getBlockPoolUsed(String bpid) throws IOException {
-    synchronized(statsLock) {
     return volumes.getBlockPoolUsed(bpid);
   }
-  }
   
   /**
    * Return true - if there are still valid volumes on the DataNode. 
@@ -611,20 +604,16 @@ public boolean hasEnoughResource() {
    */
   @Override // FSDatasetMBean
   public long getCapacity() throws IOException {
-    synchronized(statsLock) {
     return volumes.getCapacity();
   }
-  }
 
   /**
    * Return how many bytes can still be stored in the FSDataset
    */
   @Override // FSDatasetMBean
   public long getRemaining() throws IOException {
-    synchronized(statsLock) {
     return volumes.getRemaining();
   }
-  }
 
   /**
    * Return the number of failed volumes in the FSDataset.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsVolumeImpl.java

@@ -51,7 +51,6 @@
 import org.apache.hadoop.hdfs.server.datanode.checker.VolumeCheckResult;
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.DataNodeVolumeMetrics;
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsDatasetSpi;
-import org.apache.hadoop.util.AutoCloseableLock;
 import org.apache.hadoop.hdfs.DFSConfigKeys;
 import org.apache.hadoop.hdfs.DFSUtilClient;
 import org.apache.hadoop.hdfs.protocol.Block;
@@ -342,7 +341,9 @@ void onMetaFileDeletion(String bpid, long value) {
 
   private void decDfsUsedAndNumBlocks(String bpid, long value,
                                       boolean blockFileDeleted) {
-    try(AutoCloseableLock lock = dataset.acquireDatasetLock()) {
+    // BlockPoolSlice map is thread safe, and update the space used or
+    // number of blocks are atomic operations, so it doesn't require to
+    // hold the dataset lock.
     BlockPoolSlice bp = bpSlices.get(bpid);
     if (bp != null) {
       bp.decDfsUsed(value);
@@ -351,35 +352,28 @@ private void decDfsUsedAndNumBlocks(String bpid, long value,
       }
     }
   }
-  }
 
   void incDfsUsedAndNumBlocks(String bpid, long value) {
-    try(AutoCloseableLock lock = dataset.acquireDatasetLock()) {
     BlockPoolSlice bp = bpSlices.get(bpid);
     if (bp != null) {
       bp.incDfsUsed(value);
       bp.incrNumBlocks();
     }
   }
-  }
 
   void incDfsUsed(String bpid, long value) {
-    try(AutoCloseableLock lock = dataset.acquireDatasetLock()) {
     BlockPoolSlice bp = bpSlices.get(bpid);
     if (bp != null) {
       bp.incDfsUsed(value);
     }
   }
-  }
 
   @VisibleForTesting
   public long getDfsUsed() throws IOException {
     long dfsUsed = 0;
-    try(AutoCloseableLock lock = dataset.acquireDatasetLock()) {
     for(BlockPoolSlice s : bpSlices.values()) {
       dfsUsed += s.getDfsUsed();
     }
-    }
     return dfsUsed;
   }