HADOOP-1300. Improve removal of excess block replicas to be rack-aware. Contributed by Hairong.

git-svn-id: https://svn.apache.org/repos/asf/lucene/hadoop/trunk@547800 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -140,6 +140,10 @@ Trunk (unreleased changes)
  44. HADOOP-1482.  Fix secondary namenode to roll info port.
      (Dhruba Borthakur via cutting)
 
+ 45. HADOOP-1300.  Improve removal of excess block replicas to be
+     rack-aware.  Attempts are now made to keep replicas on more
+     racks.  (Hairong Kuang via cutting)
+
 
 Release 0.13.0 - 2007-06-08
 

src/java/org/apache/hadoop/dfs/FSNamesystem.java

@@ -29,6 +29,7 @@ import org.apache.hadoop.ipc.Server;
 
 import java.io.*;
 import java.util.*;
+import java.util.Map.Entry;
 
 /***************************************************
  * FSNamesystem does the actual bookkeeping work for the
@@ -2179,18 +2180,57 @@ class FSNamesystem implements FSConstants {
    *
    * srcNodes.size() - dstNodes.size() == replication
    *
-   * We pick node with least free space
-   * In the future, we might enforce some kind of policy 
-   * (like making sure replicates are spread across racks).
+   * We pick node that make sure that replicas are spread across racks and
+   * also try hard to pick one with least free space.
+   * The algorithm is first to pick a node with least free space from nodes
+   * that are on a rack holding more than one replicas of the block.
+   * So removing such a replica won't remove a rack. 
+   * If no such a node is available,
+   * then pick a node with least free space
    */
   void chooseExcessReplicates(Collection<DatanodeDescriptor> nonExcess, 
                               Block b, short replication) {
+    // first form a rack to datanodes map and
+    HashMap<String, ArrayList<DatanodeDescriptor>> rackMap =
+      new HashMap<String, ArrayList<DatanodeDescriptor>>();
+    for (Iterator<DatanodeDescriptor> iter = nonExcess.iterator();
+         iter.hasNext();) {
+      DatanodeDescriptor node = iter.next();
+      String rackName = node.getNetworkLocation();
+      ArrayList<DatanodeDescriptor> datanodeList = rackMap.get(rackName);
+      if(datanodeList==null) {
+        datanodeList = new ArrayList<DatanodeDescriptor>();
+      }
+      datanodeList.add(node);
+      rackMap.put(rackName, datanodeList);
+    }
+    
+    // split nodes into two sets
+    // priSet contains nodes on rack with more than one replica
+    // remains contains the remaining nodes
+    ArrayList<DatanodeDescriptor> priSet = new ArrayList<DatanodeDescriptor>();
+    ArrayList<DatanodeDescriptor> remains = new ArrayList<DatanodeDescriptor>();
+    for( Iterator<Entry<String, ArrayList<DatanodeDescriptor>>> iter = 
+      rackMap.entrySet().iterator(); iter.hasNext(); ) {
+      Entry<String, ArrayList<DatanodeDescriptor>> rackEntry = iter.next();
+      ArrayList<DatanodeDescriptor> datanodeList = rackEntry.getValue(); 
+      if( datanodeList.size() == 1 ) {
+        remains.add(datanodeList.get(0));
+      } else {
+        priSet.addAll(datanodeList);
+      }
+    }
+    
+    // pick one node with least space from priSet if it is not empty
+    // otherwise one node with least space from remains
     while (nonExcess.size() - replication > 0) {
       DatanodeInfo cur = null;
       long minSpace = Long.MAX_VALUE;
-            
-      for (Iterator<DatanodeDescriptor> iter = nonExcess.iterator(); iter.hasNext();) {
-        DatanodeInfo node = iter.next();
+
+      Iterator<DatanodeDescriptor> iter = 
+        priSet.isEmpty() ? remains.iterator() : priSet.iterator();
+      while( iter.hasNext() ) {
+        DatanodeDescriptor node = iter.next();
         long free = node.getRemaining();
                 
         if (minSpace > free) {
@@ -2198,7 +2238,24 @@ class FSNamesystem implements FSConstants {
           cur = node;
         }
       }
-            
+
+      // adjust rackmap, priSet, and remains
+      String rack = cur.getNetworkLocation();
+      ArrayList<DatanodeDescriptor> datanodes = rackMap.get(rack);
+      datanodes.remove(cur);
+      if(datanodes.isEmpty()) {
+        rackMap.remove(rack);
+      }
+      if (priSet.isEmpty()) {
+        remains.remove(cur);
+      } else {
+        priSet.remove(cur);
+        if (datanodes.size() == 1) {
+          priSet.remove(datanodes.get(0));
+          remains.add(datanodes.get(0));
+        }
+      }
+
       nonExcess.remove(cur);
 
       Collection<Block> excessBlocks = excessReplicateMap.get(cur.getStorageID());