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https://issues.apache.org/jira/browse/HDFS-9023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16297535#comment-16297535
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Xiao Chen commented on HDFS-9023:
---------------------------------

Hi [~surendrasingh],

Thanks for creating the jira and express your thoughts.

I ran into the same (lack of) logging issue while debugging some EC stuff, and 
did a patch locally. Git blaming to HDFS-8946, I saw your comment there and 
followed the trace to this jira. I like your idea about extra info, and 
implemented it in HDFS-12726. Would you have time to take a look? I can also 
post here and close HDFS-12726 as a dup if you don't mind. Sorry I didn't find 
this jira earlier.

> 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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