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Date: Thu, 3 Apr 2008 18:47:27 -0700 (PDT)
From: "Raghu Angadi (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3164) Use FileChannel.transferTo() when
 data is read from DataNode.
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    [ https://issues.apache.org/jira/browse/HADOOP-3164?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12585370#action_12585370 ] 

Raghu Angadi commented on HADOOP-3164:
--------------------------------------


bummer... transferTo does not handle non-blocking sockets well. I have a patch that works fine when the socket channel is in blocking mode. JavaDoc clearly indicates it can handle non-blocking sockets!. One hack is to interprete IOException and look for "Resource temporarily" at the beginning of the message.

I get the following with non-blocking sockets :
{noformat}
java.io.IOException: Resource temporarily unavailable
        at sun.nio.ch.FileChannelImpl.transferTo0(Native Method)
        at sun.nio.ch.FileChannelImpl.transferToDirectly(FileChannelImpl.java:418)
        at sun.nio.ch.FileChannelImpl.transferTo(FileChannelImpl.java:519)
        at org.apache.hadoop.dfs.DataNode.transferToFully(DataNode.java:1500)
        at org.apache.hadoop.dfs.DataNode.access$900(DataNode.java:84)
        at org.apache.hadoop.dfs.DataNode$BlockSender.sendChunks(DataNode.java:1807)
        at org.apache.hadoop.dfs.DataNode$BlockSender.sendBlock(DataNode.java:1880)
        at org.apache.hadoop.dfs.DataNode$DataXceiver.readBlock(DataNode.java:1032)
        at org.apache.hadoop.dfs.DataNode$DataXceiver.run(DataNode.java:961)
        at java.lang.Thread.run(Thread.java:619)
{noformat}

Looks like all it needs to do is just to look for EAGAIN from the system call.

> Use FileChannel.transferTo() when data is read from DataNode.
> -------------------------------------------------------------
>
>                 Key: HADOOP-3164
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3164
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>
> HADOOP-2312 talks about using FileChannel's [{{transferTo()}}|http://java.sun.com/javase/6/docs/api/java/nio/channels/FileChannel.html#transferTo(long,%20long,%20java.nio.channels.WritableByteChannel)] and [{{transferFrom()}}|http://java.sun.com/javase/6/docs/api/java/nio/channels/FileChannel.html#transferFrom(java.nio.channels.ReadableByteChannel,%20long,%20long)] in DataNode. 
> At the time DataNode neither used NIO sockets nor wrote large chunks of contiguous block data to socket. Hadoop 0.17 does both when data is seved to clients (and other datanodes). I am planning to try using transferTo() in the trunk. This might reduce DataNode's cpu by another 50% or more.
> Once HADOOP-1702 is committed, we can look into using transferFrom().

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