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https://issues.apache.org/jira/browse/HDFS-2139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15127543#comment-15127543
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M. C. Srivas commented on HDFS-2139:
------------------------------------

Just a curiosity ... why was this method chosen? Why not simply implement 
hard-links at the HDFS file level? Then a simple NN transaction is sufficient 
to create the new pathname and add a ref count to the Inode. What is the 
benefit of doing it this way? Is it because we are going across two different 
NNs? If so, how do you prevent a simultaneous delete of the file at the srcNN 
from running ahead and removing some of the blocks of the src file?


> Fast copy for HDFS.
> -------------------
>
>                 Key: HDFS-2139
>                 URL: https://issues.apache.org/jira/browse/HDFS-2139
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Pritam Damania
>            Assignee: Rituraj
>         Attachments: HDFS-2139-For-2.7.1.patch, HDFS-2139.patch, 
> HDFS-2139.patch
>
>   Original Estimate: 168h
>  Remaining Estimate: 168h
>
> There is a need to perform fast file copy on HDFS. The fast copy mechanism 
> for a file works as
> follows :
> 1) Query metadata for all blocks of the source file.
> 2) For each block 'b' of the file, find out its datanode locations.
> 3) For each block of the file, add an empty block to the namesystem for
> the destination file.
> 4) For each location of the block, instruct the datanode to make a local
> copy of that block.
> 5) Once each datanode has copied over its respective blocks, they
> report to the namenode about it.
> 6) Wait for all blocks to be copied and exit.
> This would speed up the copying process considerably by removing top of
> the rack data transfers.
> Note : An extra improvement, would be to instruct the datanode to create a
> hardlink of the block file if we are copying a block on the same datanode



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