HDFS DATANODE 磁盘容量的最小值

原创，转载请注明出处

HDFS的DATANODE的剩余空间具体要到多大？关于这个问题，下面记录下对这个问题的调查

昨天，讨论群里面给出了一个异常：

op@odbtest bin]$ hadoop fs -put ../tmp/file3 /user/hadoop/in2
14/01/15 02:14:09 WARN hdfs.DFSClient: DataStreamer Exception
org.apache.hadoop.ipc.RemoteException(java.io.IOException): File /user/hadoop/in2/file3._COPYING_ could only be replicated to 0 nodes instead of minReplication (=1).  There are 1 datanode(s) running and no node(s) are excluded in this operation.
        at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget(BlockManager.java:1384)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:2477)
        at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:555)
        at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:387)
        at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java:59582)
        at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:585)
        at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:928)
        at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2048)
        at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2044)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:396)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
        at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2042)

该异常的只在NN的日志中抛出，而DN中没有相关内容，这说明，这是在NN进行block分配的时候做了检查。

这种情况一般都是DATANODE 变成dead，或者是datanode的磁盘容量不够了。

所以建议问题提出者，给DN的datadir空出一部分空间之后，操作正常


但是，该问题的提出者，给出report 数据：

[hadoop@odbtest bin]$ hdfs dfsadmin -report
Configured Capacity: 8210259968 (7.65 GB)
Present Capacity: 599728128 (571.95 MB)
DFS Remaining: 599703552 (571.92 MB)
DFS Used: 24576 (24 KB)
DFS Used%: 0.00%
Under replicated blocks: 0
Blocks with corrupt replicas: 0
Missing blocks: 0

-------------------------------------------------
Datanodes available: 1 (1 total, 0 dead)

Live datanodes:
Name: 192.168.136.128:50010 (odbtest)
Hostname: odbtest
Decommission Status : Normal
Configured Capacity: 8210259968 (7.65 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 7610531840 (7.09 GB)
DFS Remaining: 599703552 (571.92 MB)
DFS Used%: 0.00%
DFS Remaining%: 7.30%
Last contact: Tue Jan 14 23:47:26 PST 2014

按照report的数据DFS还剩下(571.92 MB)的大小，应该是可以创建的，但是抛出了这个异常，肯定是对DATANODE的剩余最小容量做了限制。查了一下HADOOP 2.2.0的源码，
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault的方法isGoodTarget中，会对DATANODE的剩余容量进行判断：

  long remaining = node.getRemaining() - 
                     (node.getBlocksScheduled() * blockSize); 
    // check the remaining capacity of the target machine
    if (blockSize* HdfsConstants.MIN_BLOCKS_FOR_WRITE>remaining) {
      if(LOG.isDebugEnabled()) {
        threadLocalBuilder.get().append(node.toString()).append(": ")
          .append("Node ").append(NodeBase.getPath(node))
          .append(" is not chosen because the node does not have enough space ");
      }
      return false;
    }

代码中说了，当剩余容量小于blockSize* HdfsConstants.MIN_BLOCKS_FOR_WRITE的时候，会返回false，而默认情况下 blockSize* HdfsConstants.MIN_BLOCKS_FOR_WRITE=128M*5=640M> 571.92 MB,这就解释了这个异常发生的原因。