Fix append action failure

[littlezhou]

append -length 100 -file /src/t2

Log

Action starts at Wed Jun 06 10:40:54 CST 2018 : Read /src/t2
Append to /src/t2
java.io.IOException: Failed to replace a bad datanode on the existing pipeline due to no more good datanodes being available to try. (Nodes: current=[DatanodeInfoWithStorage[10.239.12.140:50010,DS-d2efbea0-5fb7-46fa-8b16-6c54364fe67c,DISK]], original=[DatanodeInfoWithStorage[10.239.12.140:50010,DS-d2efbea0-5fb7-46fa-8b16-6c54364fe67c,DISK]]). The current failed datanode replacement policy is DEFAULT, and a client may configure this via 'dfs.client.block.write.replace-datanode-on-failure.policy' in its configuration.
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.findNewDatanode(DFSOutputStream.java:925)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.addDatanode2ExistingPipeline(DFSOutputStream.java:988)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:1156)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:454)

[qiyuangong]

Will check details in local env.
I guess this issue is caused by replication number greater than living datanode. For example, replication=3, while datanode number is 2.