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https://issues.apache.org/jira/browse/HDFS-826?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841754#action_12841754
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dhruba borthakur commented on HDFS-826:
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Hi Hairong, Thanks for the review. I would really like to continue with the
approach of making getNumCurrentReplicas return the default replication factor
when there isn't a pipeline. One way that I am thinking is that the app can
visualize one file to be one huge ever-expanding block and as long as there are
no pipeline errors, the app will always see a NumReplicas as the default
replication factor. I checked this with a few HBase folks and it appears
simpler to them from an application programmer's perspective.
I hope this is ok with you and thanks for reviewing it.
> Allow a mechanism for an application to detect that datanode(s) have died in
> the write pipeline
> ------------------------------------------------------------------------------------------------
>
> Key: HDFS-826
> URL: https://issues.apache.org/jira/browse/HDFS-826
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs client
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
> Attachments: Replicable4.txt, ReplicableHdfs.txt,
> ReplicableHdfs2.txt, ReplicableHdfs3.txt
>
>
> HDFS does not replicate the last block of the file that is being currently
> written to by an application. Every datanode death in the write pipeline
> decreases the reliability of the last block of the currently-being-written
> block. This situation can be improved if the application can be notified of a
> datanode death in the write pipeline. Then, the application can decide what
> is the right course of action to be taken on this event.
> In our use-case, the application can close the file on the first datanode
> death, and start writing to a newly created file. This ensures that the
> reliability guarantee of a block is close to 3 at all time.
> One idea is to make DFSOutoutStream. write() throw an exception if the number
> of datanodes in the write pipeline fall below minimum.replication.factor that
> is set on the client (this is backward compatible).
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