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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4953) enable HDFS local reads via mmap
Date Fri, 16 Aug 2013 18:30:50 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13742498#comment-13742498
] 

Colin Patrick McCabe commented on HDFS-4953:
--------------------------------------------

I'll add a config variable for the number of times per expiration period the cleaner thread
should run.

bq. javac warnings still need to be addressed (looks like via the ignore file)

There is no ignore file for javac warnings.  test-patch.sh just has an awk script that compares
the number after the patch to the number before.  Unfortunately, we don't have any way to
avoid these warnings in the munmap.

Since there's already a lot of javadoc, let's defer adding more documentation to another JIRA.
 An .apt.vm file might be the right place for this.
                
> enable HDFS local reads via mmap
> --------------------------------
>
>                 Key: HDFS-4953
>                 URL: https://issues.apache.org/jira/browse/HDFS-4953
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>    Affects Versions: 2.3.0
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>         Attachments: benchmark.png, HDFS-4953.001.patch, HDFS-4953.002.patch, HDFS-4953.003.patch,
HDFS-4953.004.patch, HDFS-4953.005.patch, HDFS-4953.006.patch, HDFS-4953.007.patch
>
>
> Currently, the short-circuit local read pathway allows HDFS clients to access files directly
without going through the DataNode.  However, all of these reads involve a copy at the operating
system level, since they rely on the read() / pread() / etc family of kernel interfaces.
> We would like to enable HDFS to read local files via mmap.  This would enable truly zero-copy
reads.
> In the initial implementation, zero-copy reads will only be performed when checksums
were disabled.  Later, we can use the DataNode's cache awareness to only perform zero-copy
reads when we know that checksum has already been verified.

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