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https://issues.apache.org/jira/browse/HADOOP-15292?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16388934#comment-16388934
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Íñigo Goiri commented on HADOOP-15292:
--------------------------------------

Thanks [~virajith] and [~chris.douglas].
I missed the last part:
{code}
assertCounter(readCounter, readsFromClient + numFiles, rb);
{code}
I think that's actually enough.
Just to double check, before the change, the unit test failed in this assert, 
right?

> Distcp's use of pread is slowing it down.
> -----------------------------------------
>
>                 Key: HADOOP-15292
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15292
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: tools/distcp
>    Affects Versions: 3.0.0
>            Reporter: Virajith Jalaparti
>            Priority: Minor
>         Attachments: HADOOP-15292.000.patch, HADOOP-15292.001.patch
>
>
> Distcp currently uses positioned-reads (in 
> RetriableFileCopyCommand#copyBytes) when the source offset is > 0. This 
> results in unnecessary overheads (new BlockReader being created on the 
> client-side, multiple readBlock() calls to the Datanodes, each of which 
> requires the creation of a BlockSender and an inputstream to the ReplicaInfo).



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