HDFS dead datanode检测机制

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HDFS NameNode是通过心跳(heartbeat)来判断DataNode是否存活的，每个DataNode默认每个3s会发送一条心跳信息给NameNode，心跳发送间隔时间是通过dfs.heartbeat.interval来配置。但当某个DataNode挂了，默认情况下，NameNode往往要过了10分钟才会把这个节点从存活节点(live node)中移除，并标记为dead。那这时如果拷贝文件到HDFS，正好分配到这台挂了的DataNode，客户端会收到异常错误，错误如下:

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2020-09-02 09:59:11,023 INFO hdfs.DataStreamer: Exception in createBlockOutputStream blk_1073741826_1002
java.net.ConnectException: Connection refused
	at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
	at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:714)
	at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206)
	at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:533)
	at org.apache.hadoop.hdfs.DataStreamer.createSocketForPipeline(DataStreamer.java:253)
	at org.apache.hadoop.hdfs.DataStreamer.createBlockOutputStream(DataStreamer.java:1725)
	at org.apache.hadoop.hdfs.DataStreamer.nextBlockOutputStream(DataStreamer.java:1679)
	at org.apache.hadoop.hdfs.DataStreamer.run(DataStreamer.java:716)
2020-09-02 09:59:11,026 WARN hdfs.DataStreamer: Abandoning BP-284823762-192.168.122.11-1599788894679:blk_1073741826_1002
2020-09-02 09:59:11,041 WARN hdfs.DataStreamer: Excluding datanode DatanodeInfoWithStorage[192.168.122.243:9866,DS-fc5a0994-ca09-453e-850d-79ec51882715,DISK]
2020-09-02 09:59:11,073 WARN hdfs.DataStreamer: DataStreamer Exception
org.apache.hadoop.ipc.RemoteException(java.io.IOException): File /resolv.conf._COPYING_ could only be written to 0 of the 1 minReplication nodes. There are 1 datanode(s) running and 1 node(s) are excluded in this operation.
	at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget4NewBlock(BlockManager.java:2219)
	at org.apache.hadoop.hdfs.server.namenode.FSDirWriteFileOp.chooseTargetForNewBlock(FSDirWriteFileOp.java:294)
	at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:2789)
	at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:892)
	at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:574)
	at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:528)
	at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1070)
	at org.apache.hadoop.ipc.Server$RpcCall.run(Server.java:999)
	at org.apache.hadoop.ipc.Server$RpcCall.run(Server.java:927)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:422)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1730)
	at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2915)

	at org.apache.hadoop.ipc.Client.getRpcResponse(Client.java:1545)
	at org.apache.hadoop.ipc.Client.call(Client.java:1491)
	at org.apache.hadoop.ipc.Client.call(Client.java:1388)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:233)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:118)
	at com.sun.proxy.$Proxy9.addBlock(Unknown Source)
	at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.addBlock(ClientNamenodeProtocolTranslatorPB.java:517)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:422)
	at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invokeMethod(RetryInvocationHandler.java:165)
	at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invoke(RetryInvocationHandler.java:157)
	at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invokeOnce(RetryInvocationHandler.java:95)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:359)
	at com.sun.proxy.$Proxy10.addBlock(Unknown Source)
	at org.apache.hadoop.hdfs.DFSOutputStream.addBlock(DFSOutputStream.java:1081)
	at org.apache.hadoop.hdfs.DataStreamer.locateFollowingBlock(DataStreamer.java:1866)
	at org.apache.hadoop.hdfs.DataStreamer.nextBlockOutputStream(DataStreamer.java:1668)
	at org.apache.hadoop.hdfs.DataStreamer.run(DataStreamer.java:716)
put: File /resolv.conf._COPYING_ could only be written to 0 of the 1 minReplication nodes. There are 1 datanode(s) running and 1 node(s) are excluded in this operation.

IP为192.168.122.243的DataNode是挂了的机器，当我们使用命令hdfs dfs -put /etc/resolv.conf /写入文件时，会报以上错误。在实际应用中，NameNode 10分钟之后才把dead DataNode移除并标记为dead，这个时间有点过长。通过查看NameNode页面或者命令hdfs dfsadmin -report获取DataNode信息：

这个由NameNode确定DataNode dead的时长由以下公式计算出来：

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2 * dfs.namenode.heartbeat.recheck-interval + 10 * 1000 * dfs.heartbeat.interval

dfs.namenode.heartbeat.recheck-interval 默认是300000ms(5min)，dfs.heartbeat.interval 默认3s，所有实际上这个时长为10min 30s。

所以，如果要降低NameNode标记故障的的DataNode为dead的时长，dfs.namenode.heartbeat.recheck-interval 可以适当配小点:

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<property>
    <name>dfs.namenode.heartbeat.recheck-interval</name>
    <value>15000</value>
</property>

参考