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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5182) BlockReaderLocal must allow zero-copy reads only when the DN believes it's valid
Date Wed, 05 Feb 2014 01:01:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13891547#comment-13891547
] 

Colin Patrick McCabe commented on HDFS-5182:
--------------------------------------------

bq. Okay now it looks more clear to me now. Thanks for the explanation.

Glad to be helpful.

bq. My bad. I mixed Linux with SunOS. You can do it using sendmsg() / recvmsg() as you mentioned
in the previous comments.

I didn't realize that ioctl was the way to do this under SunOS.  Interesting.  Sending fds
via {{sendmsg}} seems to work on all the modern UNIX variants, so I think that we're good
there.  On Windows, we'll need to use {{DuplicateHandle}}.

> BlockReaderLocal must allow zero-copy  reads only when the DN believes it's valid
> ---------------------------------------------------------------------------------
>
>                 Key: HDFS-5182
>                 URL: https://issues.apache.org/jira/browse/HDFS-5182
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>
> BlockReaderLocal must allow zero-copy reads only when the DN believes it's valid.  This
implies adding a new field to the response to REQUEST_SHORT_CIRCUIT_FDS.  We also need some
kind of heartbeat from the client to the DN, so that the DN can inform the client when the
mapped region is no longer locked into memory.



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