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From "Prasanth Jayachandran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-11945) ORC with non-local reads may not be reusing connection to DN
Date Fri, 25 Sep 2015 17:57:07 GMT

    [ https://issues.apache.org/jira/browse/HIVE-11945?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14908413#comment-14908413
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Prasanth Jayachandran commented on HIVE-11945:
----------------------------------------------

[~rajesh.balamohan] Thanks for the S3 analysis. Can you link the new HDFS jira to this patch?


The latest patch LGTM as well, +1

> ORC with non-local reads may not be reusing connection to DN
> ------------------------------------------------------------
>
>                 Key: HIVE-11945
>                 URL: https://issues.apache.org/jira/browse/HIVE-11945
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 1.3.0, 2.0.0
>            Reporter: Rajesh Balamohan
>            Assignee: Rajesh Balamohan
>         Attachments: HIVE-11945.1.patch, HIVE-11945.2.patch, HIVE-11945.3.patch
>
>
> When “seek + readFully(buffer, offset, length)” is used,  DFSInputStream ends up
going via “readWithStrategy()”.  This sets up BlockReader with length equivalent to that
of the block size. So until this position is reached, RemoteBlockReader2.peer would not be
added to the PeerCache (Plz refer RemoteBlockReader2.close() in HDFS).  So eventually the
next call to the same DN would end opening a new socket.  In ORC, when it is not a data local
read, this has a the possibility of opening/closing lots of connections with DN.  
> In random reads, it would be good to set this length to the amount of data that is to
be read (e.g pread call in DFSInputStream which sets up the BlockReader’s length correctly
& the code path returns the Peer back to peer cache properly).  “readFully(position,
buffer, offset, length)” follows this code path and ends up reusing the connections properly.
Creating this JIRA to fix this issue.



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