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From "Tsuyoshi OZAWA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4953) enable HDFS local reads via mmap
Date Wed, 31 Jul 2013 08:27:52 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13725019#comment-13725019
] 

Tsuyoshi OZAWA commented on HDFS-4953:
--------------------------------------

Thanks for your work, Colin.

I looked your patch. Now this patch uses the reference-counter based memory management strategy.
Users, therefore, need to manage cursors and have the risk of memory leak. PhantomReference-based
memory management strategy can prevent users from memory leak, because we can hook cursors
which is unreferenced but still open based on GC's reference. We can do this in another JIRA.

http://docs.oracle.com/javase/7/docs/api/java/lang/ref/PhantomReference.html
                
> 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
>
>
> 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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