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From "Kihwal Lee (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-281) Explore usage of the sendfile api via java.nio.channels.FileChannel.transfer{To|From} for i/o in datanodes
Date Mon, 14 Mar 2011 19:01:29 GMT

    [ https://issues.apache.org/jira/browse/HDFS-281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13006574#comment-13006574
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Kihwal Lee commented on HDFS-281:
---------------------------------

I believe it was fixed by HADOOP-3164, which was committed on 4/25/2008.

> Explore usage of the sendfile api via java.nio.channels.FileChannel.transfer{To|From}
for i/o in datanodes
> ----------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-281
>                 URL: https://issues.apache.org/jira/browse/HDFS-281
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Arun C Murthy
>
> We could potentially gain a lot of performance by using the *sendfile* system call:
> $ man sendfile
> {noformat}
> DESCRIPTION
>        This  call  copies  data between one file descriptor and another.  Either or both
of these file descriptors may refer to a socket (but see below).
>        in_fd should be a file descriptor opened for reading and out_fd should be a descriptor
opened for writing.  offset is  a  pointer  to  a  variable
>        holding  the input file pointer position from which sendfile() will start reading
data.  When sendfile() returns, this variable will be set to the
>        offset of the byte following the last byte that was read.  count is the number
of bytes to copy between file descriptors.
>        Because this copying is done within the kernel, sendfile() does not need to spend
time transferring data to and from user space.
> {noformat}
> The nio package offers this via the java.nio.channels.FileChannel.transfer{To|From} apis:
> http://java.sun.com/j2se/1.5.0/docs/api/java/nio/channels/FileChannel.html#transferFrom(java.nio.channels.ReadableByteChannel,%20long,%20long)
> http://java.sun.com/j2se/1.5.0/docs/api/java/nio/channels/FileChannel.html#transferTo(long,%20long,%20java.nio.channels.WritableByteChannel)
> From the javadocs:
> {noformat}
>      This method is potentially much more efficient than a simple loop that reads from
this channel and writes to the target channel. Many operating systems can transfer bytes directly
from the filesystem cache to the target channel without actually copying them.
> {noformat}
> ----
> Hence, this could well-worth exploring for doing io at the datanodes...

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