HDFS-15160. ReplicaMap, Disk Balancer, Directory Scanner and various FsDatasetImpl methods should use datanode readlock. Contributed by Stephen O'Donnell.

Signed-off-by: Wei-Chiu Chuang <weichiu@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java

@@ -606,6 +606,10 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
   public static final String DFS_DATANODE_LOCK_FAIR_KEY =
       "dfs.datanode.lock.fair";
   public static final boolean DFS_DATANODE_LOCK_FAIR_DEFAULT = true;
+  public static final String DFS_DATANODE_LOCK_READ_WRITE_ENABLED_KEY =
+      "dfs.datanode.lock.read.write.enabled";
+  public static final Boolean DFS_DATANODE_LOCK_READ_WRITE_ENABLED_DEFAULT =
+      true;
   public static final String  DFS_DATANODE_LOCK_REPORTING_THRESHOLD_MS_KEY =
       "dfs.datanode.lock-reporting-threshold-ms";
   public static final long

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockSender.java

@@ -255,7 +255,7 @@ class BlockSender implements java.io.Closeable {
       // the append write.
       ChunkChecksum chunkChecksum = null;
       final long replicaVisibleLength;
-      try(AutoCloseableLock lock = datanode.data.acquireDatasetLock()) {
+      try(AutoCloseableLock lock = datanode.data.acquireDatasetReadLock()) {
         replica = getReplica(block, datanode);
         replicaVisibleLength = replica.getVisibleLength();
       }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java

@@ -3060,7 +3060,7 @@ void transferReplicaForPipelineRecovery(final ExtendedBlock b,
     final BlockConstructionStage stage;
 
     //get replica information
-    try(AutoCloseableLock lock = data.acquireDatasetLock()) {
+    try(AutoCloseableLock lock = data.acquireDatasetReadLock()) {
       Block storedBlock = data.getStoredBlock(b.getBlockPoolId(),
           b.getBlockId());
       if (null == storedBlock) {

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DirectoryScanner.java

@@ -473,7 +473,7 @@ private void scan() {
     blockPoolReport.sortBlocks();
 
     // Hold FSDataset lock to prevent further changes to the block map
-    try (AutoCloseableLock lock = dataset.acquireDatasetLock()) {
+    try (AutoCloseableLock lock = dataset.acquireDatasetReadLock()) {
       for (final String bpid : blockPoolReport.getBlockPoolIds()) {
         List<ScanInfo> blockpoolReport = blockPoolReport.getScanInfo(bpid);
 

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DiskBalancer.java

@@ -504,7 +504,7 @@ private Map<String, String> getStorageIDToVolumeBasePathMap()
     Map<String, String> storageIDToVolBasePathMap = new HashMap<>();
     FsDatasetSpi.FsVolumeReferences references;
     try {
-      try(AutoCloseableLock lock = this.dataset.acquireDatasetLock()) {
+      try(AutoCloseableLock lock = this.dataset.acquireDatasetReadLock()) {
         references = this.dataset.getFsVolumeReferences();
         for (int ndx = 0; ndx < references.size(); ndx++) {
           FsVolumeSpi vol = references.get(ndx);

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

@@ -657,12 +657,16 @@ ReplicaInfo moveBlockAcrossVolumes(final ExtendedBlock block,
       FsVolumeSpi destination) throws IOException;
 
   /**
-   * Acquire the lock of the data set.
+   * Acquire the lock of the data set. This prevents other threads from
+   * modifying the volume map structure inside the datanode, but other changes
+   * are still possible. For example modifying the genStamp of a block instance.
    */
   AutoCloseableLock acquireDatasetLock();
 
   /***
-   * Acquire the read lock of the data set.
+   * Acquire the read lock of the data set. This prevents other threads from
+   * modifying the volume map structure inside the datanode, but other changes
+   * are still possible. For example modifying the genStamp of a block instance.
    * @return The AutoClosable read lock instance.
    */
   AutoCloseableLock acquireDatasetReadLock();

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

@@ -183,7 +183,7 @@ public StorageReport[] getStorageReports(String bpid)
 
   @Override
   public FsVolumeImpl getVolume(final ExtendedBlock b) {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final ReplicaInfo r =
           volumeMap.get(b.getBlockPoolId(), b.getLocalBlock());
       return r != null ? (FsVolumeImpl) r.getVolume() : null;
@@ -193,7 +193,7 @@ public FsVolumeImpl getVolume(final ExtendedBlock b) {
   @Override // FsDatasetSpi
   public Block getStoredBlock(String bpid, long blkid)
       throws IOException {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       ReplicaInfo r = volumeMap.get(bpid, blkid);
       if (r == null) {
         return null;
@@ -206,7 +206,7 @@ public Block getStoredBlock(String bpid, long blkid)
   public Set<? extends Replica> deepCopyReplica(String bpid)
       throws IOException {
     Set<? extends Replica> replicas = null;
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       replicas = new HashSet<>(volumeMap.replicas(bpid) == null ? Collections.
           EMPTY_SET : volumeMap.replicas(bpid));
     }
@@ -302,7 +302,20 @@ public LengthInputStream getMetaDataInputStream(ExtendedBlock b)
             DFSConfigKeys.DFS_DATANODE_LOCK_REPORTING_THRESHOLD_MS_DEFAULT,
             TimeUnit.MILLISECONDS));
     this.datasetWriteLock = new AutoCloseableLock(datasetRWLock.writeLock());
+    boolean enableRL = conf.getBoolean(
+        DFSConfigKeys.DFS_DATANODE_LOCK_READ_WRITE_ENABLED_KEY,
+        DFSConfigKeys.DFS_DATANODE_LOCK_READ_WRITE_ENABLED_DEFAULT);
+    // The read lock can be disabled by the above config key. If it is disabled
+    // then we simply make the both the read and write lock variables hold
+    // the write lock. All accesses to the lock are via these variables, so that
+    // effectively disables the read lock.
+    if (enableRL) {
+      LOG.info("The datanode lock is a read write lock");
       this.datasetReadLock = new AutoCloseableLock(datasetRWLock.readLock());
+    } else {
+      LOG.info("The datanode lock is an exclusive write lock");
+      this.datasetReadLock = this.datasetWriteLock;
+    }
     this.datasetWriteLockCondition = datasetWriteLock.newCondition();
 
     // The number of volumes required for operation is the total number
@@ -342,7 +355,7 @@ public LengthInputStream getMetaDataInputStream(ExtendedBlock b)
     }
 
     storageMap = new ConcurrentHashMap<String, DatanodeStorage>();
-    volumeMap = new ReplicaMap(datasetRWLock);
+    volumeMap = new ReplicaMap(datasetReadLock, datasetWriteLock);
     ramDiskReplicaTracker = RamDiskReplicaTracker.getInstance(conf, this);
 
     @SuppressWarnings("unchecked")
@@ -475,7 +488,8 @@ private void addVolume(Storage.StorageDirectory sd) throws IOException {
                               .setConf(this.conf)
                               .build();
     FsVolumeReference ref = fsVolume.obtainReference();
-    ReplicaMap tempVolumeMap = new ReplicaMap(datasetRWLock);
+    ReplicaMap tempVolumeMap =
+        new ReplicaMap(datasetReadLock, datasetWriteLock);
     fsVolume.getVolumeMap(tempVolumeMap, ramDiskReplicaTracker);
 
     activateVolume(tempVolumeMap, sd, storageLocation.getStorageType(), ref);
@@ -810,7 +824,7 @@ public InputStream getBlockInputStream(ExtendedBlock b,
       long seekOffset) throws IOException {
 
     ReplicaInfo info;
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       info = volumeMap.get(b.getBlockPoolId(), b.getLocalBlock());
     }
 
@@ -898,7 +912,7 @@ ReplicaInfo getReplicaInfo(String bpid, long blkid)
   @Override // FsDatasetSpi
   public ReplicaInputStreams getTmpInputStreams(ExtendedBlock b,
       long blkOffset, long metaOffset) throws IOException {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       ReplicaInfo info = getReplicaInfo(b);
       FsVolumeReference ref = info.getVolume().obtainReference();
       try {
@@ -1023,7 +1037,7 @@ public ReplicaInfo moveBlockAcrossStorage(ExtendedBlock block,
     }
 
     FsVolumeReference volumeRef = null;
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       volumeRef = volumes.getNextVolume(targetStorageType, targetStorageId,
           block.getNumBytes());
     }
@@ -1137,7 +1151,7 @@ public ReplicaInfo moveBlockAcrossVolumes(ExtendedBlock block, FsVolumeSpi
 
     FsVolumeReference volumeRef = null;
 
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       volumeRef = destination.obtainReference();
     }
 
@@ -1930,7 +1944,7 @@ public Map<DatanodeStorage, BlockListAsLongs> getBlockReports(String bpid) {
         new HashMap<String, BlockListAsLongs.Builder>();
 
     List<FsVolumeImpl> curVolumes = null;
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       curVolumes = volumes.getVolumes();
       for (FsVolumeSpi v : curVolumes) {
         builders.put(v.getStorageID(), BlockListAsLongs.builder(maxDataLength));
@@ -1989,7 +2003,7 @@ public Map<DatanodeStorage, BlockListAsLongs> getBlockReports(String bpid) {
    */
   @Override
   public List<ReplicaInfo> getFinalizedBlocks(String bpid) {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final List<ReplicaInfo> finalized = new ArrayList<ReplicaInfo>(
           volumeMap.size(bpid));
       for (ReplicaInfo b : volumeMap.replicas(bpid)) {
@@ -2082,9 +2096,7 @@ private boolean isValid(final ExtendedBlock b, final ReplicaState state) {
   ReplicaInfo validateBlockFile(String bpid, long blockId) {
     //Should we check for metadata file too?
     final ReplicaInfo r;
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
     r = volumeMap.get(bpid, blockId);
-    }
     if (r != null) {
       if (r.blockDataExists()) {
         return r;
@@ -2327,7 +2339,7 @@ public boolean isCached(String bpid, long blockId) {
 
   @Override // FsDatasetSpi
   public boolean contains(final ExtendedBlock block) {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final long blockId = block.getLocalBlock().getBlockId();
       final String bpid = block.getBlockPoolId();
       final ReplicaInfo r = volumeMap.get(bpid, blockId);
@@ -2655,7 +2667,7 @@ public ReplicaInfo getReplica(String bpid, long blockId) {
 
   @Override 
   public String getReplicaString(String bpid, long blockId) {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final Replica r = volumeMap.get(bpid, blockId);
       return r == null ? "null" : r.toString();
     }
@@ -2882,7 +2894,7 @@ private ReplicaInfo updateReplicaUnderRecovery(
   @Override // FsDatasetSpi
   public long getReplicaVisibleLength(final ExtendedBlock block)
   throws IOException {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final Replica replica = getReplicaInfo(block.getBlockPoolId(),
           block.getBlockId());
       if (replica.getGenerationStamp() < block.getGenerationStamp()) {
@@ -3032,12 +3044,13 @@ public void deleteBlockPool(String bpid, boolean force)
   @Override // FsDatasetSpi
   public BlockLocalPathInfo getBlockLocalPathInfo(ExtendedBlock block)
       throws IOException {
-    try (AutoCloseableLock lock = datasetWriteLock.acquire()) {
+    try (AutoCloseableLock lock = datasetReadLock.acquire()) {
       final Replica replica = volumeMap.get(block.getBlockPoolId(),
           block.getBlockId());
       if (replica == null) {
         throw new ReplicaNotFoundException(block);
       }
+      synchronized(replica) {
         if (replica.getGenerationStamp() < block.getGenerationStamp()) {
           throw new IOException(
               "Replica generation stamp < block generation stamp, block="
@@ -3046,6 +3059,7 @@ public BlockLocalPathInfo getBlockLocalPathInfo(ExtendedBlock block)
           block.setGenerationStamp(replica.getGenerationStamp());
         }
       }
+    }
 
     ReplicaInfo r = getBlockReplica(block);
     File blockFile = new File(r.getBlockURI());

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

@@ -33,7 +33,6 @@
  * Maintains the replica map. 
  */
 class ReplicaMap {
-  private final ReadWriteLock rwLock;
   // Lock object to synchronize this instance.
   private final AutoCloseableLock readLock;
   private final AutoCloseableLock writeLock;
@@ -53,18 +52,22 @@ public int compare(Object o1, Object o2) {
         }
       };
 
-  ReplicaMap(ReadWriteLock lock) {
+  ReplicaMap(AutoCloseableLock readLock, AutoCloseableLock writeLock) {
-    if (lock == null) {
+    if (readLock == null || writeLock == null) {
       throw new HadoopIllegalArgumentException(
           "Lock to synchronize on cannot be null");
     }
-    this.rwLock = lock;
+    this.readLock = readLock;
-    this.readLock = new AutoCloseableLock(rwLock.readLock());
+    this.writeLock = writeLock;
-    this.writeLock = new AutoCloseableLock(rwLock.writeLock());
+  }
+
+  ReplicaMap(ReadWriteLock lock) {
+    this(new AutoCloseableLock(lock.readLock()),
+        new AutoCloseableLock(lock.writeLock()));
   }
   
   String[] getBlockPoolList() {
-    try (AutoCloseableLock l = writeLock.acquire()) {
+    try (AutoCloseableLock l = readLock.acquire()) {
       return map.keySet().toArray(new String[map.keySet().size()]);   
     }
   }
@@ -109,7 +112,7 @@ ReplicaInfo get(String bpid, Block block) {
    */
   ReplicaInfo get(String bpid, long blockId) {
     checkBlockPool(bpid);
-    try (AutoCloseableLock l = writeLock.acquire()) {
+    try (AutoCloseableLock l = readLock.acquire()) {
       FoldedTreeSet<ReplicaInfo> set = map.get(bpid);
       if (set == null) {
         return null;
@@ -235,7 +238,7 @@ ReplicaInfo remove(String bpid, long blockId) {
    * @return the number of replicas in the map
    */
   int size(String bpid) {
-    try (AutoCloseableLock l = writeLock.acquire()) {
+    try (AutoCloseableLock l = readLock.acquire()) {
       FoldedTreeSet<ReplicaInfo> set = map.get(bpid);
       return set != null ? set.size() : 0;
     }
@@ -281,4 +284,14 @@ void cleanUpBlockPool(String bpid) {
   AutoCloseableLock getLock() {
     return writeLock;
   }
+
+  /**
+   * Get the lock object used for synchronizing the ReplicasMap for read only
+   * operations.
+   * @return The read lock object
+   */
+  AutoCloseableLock getReadLock() {
+    return readLock;
+  }
+
 }

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -3250,6 +3250,19 @@
   </description>
 </property>
 
+<property>
+  <name>dfs.datanode.lock.read.write.enabled</name>
+  <value>true</value>
+  <description>If this is true, the FsDataset lock will be a read write lock. If
+    it is false, all locks will be a write lock.
+    Enabling this should give better datanode throughput, as many read only
+    functions can run concurrently under the read lock, when they would
+    previously have required the exclusive write lock. As the feature is
+    experimental, this switch can be used to disable the shared read lock, and
+    cause all lock acquisitions to use the exclusive write lock.
+  </description>
+</property>
+
 <property>
   <name>dfs.datanode.lock-reporting-threshold-ms</name>
   <value>300</value>

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java

@@ -65,6 +65,7 @@
 import org.apache.hadoop.io.MultipleIOException;
 import org.apache.hadoop.test.GenericTestUtils;
 import org.apache.hadoop.test.LambdaTestUtils;
+import org.apache.hadoop.util.AutoCloseableLock;
 import org.apache.hadoop.util.FakeTimer;
 import org.apache.hadoop.util.StringUtils;
 import org.junit.Assert;
@@ -84,6 +85,7 @@
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_DN_CACHED_DFSUSED_CHECK_INTERVAL_MS;
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_DATANODE_SCAN_PERIOD_HOURS_KEY;
@@ -198,6 +200,101 @@ public void setUp() throws IOException {
     assertEquals(0, dataset.getNumFailedVolumes());
   }
 
+  @Test
+  public void testReadLockEnabledByDefault()
+      throws IOException, InterruptedException {
+    final FsDatasetSpi ds = dataset;
+    AtomicBoolean accessed = new AtomicBoolean(false);
+    CountDownLatch latch = new CountDownLatch(1);
+    CountDownLatch waiterLatch = new CountDownLatch(1);
+
+    Thread holder = new Thread() {
+      public void run() {
+        try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
+          latch.countDown();
+          sleep(10000);
+        } catch (Exception e) {
+        }
+      }
+    };
+
+    Thread waiter = new Thread() {
+      public void run() {
+        try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
+          waiterLatch.countDown();
+          accessed.getAndSet(true);
+        } catch (Exception e) {
+        }
+      }
+    };
+
+    holder.start();
+    latch.await();
+    waiter.start();
+    waiterLatch.await();
+    // The holder thread is still holding the lock, but the waiter can still
+    // run as the lock is a shared read lock.
+    assertEquals(true, accessed.get());
+    holder.interrupt();
+    holder.join();
+    waiter.join();
+  }
+
+  @Test(timeout=10000)
+  public void testReadLockCanBeDisabledByConfig()
+      throws IOException, InterruptedException {
+    HdfsConfiguration conf = new HdfsConfiguration();
+    conf.setBoolean(
+        DFSConfigKeys.DFS_DATANODE_LOCK_READ_WRITE_ENABLED_KEY, false);
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf)
+        .numDataNodes(1).build();
+    try {
+      cluster.waitActive();
+      DataNode dn = cluster.getDataNodes().get(0);
+      final FsDatasetSpi<?> ds = DataNodeTestUtils.getFSDataset(dn);
+
+      CountDownLatch latch = new CountDownLatch(1);
+      CountDownLatch waiterLatch = new CountDownLatch(1);
+      AtomicBoolean accessed = new AtomicBoolean(false);
+
+      Thread holder = new Thread() {
+        public void run() {
+          try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
+            latch.countDown();
+            sleep(10000);
+          } catch (Exception e) {
+          }
+        }
+      };
+
+      Thread waiter = new Thread() {
+        public void run() {
+          try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
+            accessed.getAndSet(true);
+            waiterLatch.countDown();
+          } catch (Exception e) {
+          }
+        }
+      };
+
+      holder.start();
+      latch.await();
+      waiter.start();
+      Thread.sleep(200);
+      // Waiting thread should not have been able to update the variable
+      // as the read lock is disabled and hence an exclusive lock.
+      assertEquals(false, accessed.get());
+      holder.interrupt();
+      holder.join();
+      waiterLatch.await();
+      // After the holder thread exits, the variable is updated.
+      assertEquals(true, accessed.get());
+      waiter.join();
+    } finally {
+      cluster.shutdown();
+    }
+  }
+
   @Test
   public void testAddVolumes() throws IOException {
     final int numNewVolumes = 3;
@@ -244,8 +341,8 @@ public void testAddVolumes() throws IOException {
 
   @Test
   public void testAddVolumeWithSameStorageUuid() throws IOException {
-    HdfsConfiguration conf = new HdfsConfiguration();
+    HdfsConfiguration config = new HdfsConfiguration();
-    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf)
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(config)
         .numDataNodes(1).build();
     try {
       cluster.waitActive();