commit e45c053f128c27cc358c912d1ac2973bae135e46
Author: Jean-Baptiste Note <jb.note@criteo.com>
Date:   Thu Jun 27 14:48:59 2013 +0200

    Take a read lock rather than a write lock when checkpointing the image

diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
index cf5314a..b871e01 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
@@ -997,6 +997,9 @@ public class FSNamesystem implements Namesystem, FSClusterStats,
   public void readLock() {
     this.fsLock.readLock().lock();
   }
+  public void readLockInterruptibly() throws InterruptedException {
+    this.fsLock.readLock().lockInterruptibly();
+  }
   @Override
   public void readUnlock() {
     this.fsLock.readLock().unlock();
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyCheckpointer.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyCheckpointer.java
index a97e55d..4980942 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyCheckpointer.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyCheckpointer.java
@@ -144,7 +144,7 @@ public class StandbyCheckpointer {
     assert canceler != null;
     long txid;
     
-    namesystem.writeLockInterruptibly();
+    namesystem.readLockInterruptibly();
     try {
       assert namesystem.getEditLog().isOpenForRead() :
         "Standby Checkpointer should only attempt a checkpoint when " +
@@ -167,7 +167,7 @@ public class StandbyCheckpointer {
       assert txid == thisCheckpointTxId : "expected to save checkpoint at txid=" +
         thisCheckpointTxId + " but instead saved at txid=" + txid;
     } finally {
-      namesystem.writeUnlock();
+      namesystem.readUnlock();
     }
     
     // Upload the saved checkpoint back to the active