Surendra Singh Lilhore created HDFS-9023:
--------------------------------------------

             Summary: When NN is not able to identify DN for replication, 
reason behind it can be logged
                 Key: HDFS-9023
                 URL: https://issues.apache.org/jira/browse/HDFS-9023
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs-client, namenode
    Affects Versions: 2.7.1
            Reporter: Surendra Singh Lilhore
            Assignee: Surendra Singh Lilhore
            Priority: Critical


When NN is not able to identify DN for replication, reason behind it can be 
logged (at least critical information why DNs not chosen like disk is full). At 
present it is expected to enable debug log.

For example the reason for below error looks like all 7 DNs are busy for data 
writes. But at client or NN side no hint is given in the log message.
{noformat}
File /tmp/logs/spark/logs/application_1437051383180_0610/xyz-195_26009.tmp 
could only be replicated to 0 nodes instead of minReplication (=1).  There are 
7 datanode(s) running and no node(s) are excluded in this operation.
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget4NewBlock(BlockManager.java:1553)
 
{noformat}



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