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https://issues.apache.org/jira/browse/HDFS-2834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13192493#comment-13192493
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Todd Lipcon commented on HDFS-2834:
-----------------------------------
This is also useful whenever a native decompression codec is being used. In
those cases, we generally have the following copies:
1) Socket -> DirectByteBuffer (in SocketChannel implementation)
2) DirectByteBuffer -> byte[] (in SocketInputStream)
3) byte[] -> Native buffer (set up for decompression)
4*) decompression to a different native buffer (not really a copy -
decompression necessarily rewrites)
5) native buffer -> byte[]
with the proposed improvement we can hopefully eliminate #2,#3 for all
applications, and #2,#3,and #5 for libhdfs.
> ByteBuffer-based read API for DFSClient
> ---------------------------------------
>
> Key: HDFS-2834
> URL: https://issues.apache.org/jira/browse/HDFS-2834
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Henry Robinson
> Assignee: Henry Robinson
>
> The {{DFSClient}} read-path always copies bytes into a JVM-allocated
> {{byte[]}}. Although for many clients this is desired behaviour, in certain
> situations, such as native-reads through libhdfs, this imposes an extra copy
> penalty since the {{byte[]}} needs to be copied out again into a natively
> readable memory area.
> For these cases, it would be preferable to allow the client to supply its own
> buffer, wrapped in a {{ByteBuffer}}, to avoid that final copy overhead.
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