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https://issues.apache.org/jira/browse/HDFS-2834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13223023#comment-13223023
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dhruba borthakur commented on HDFS-2834:
----------------------------------------
In our own variant of hdfs, we have introduced a scatter-gather api in
FSDataInputStream:
{code}
public List<ByteBuffer> readFullyScatterGather(long position, int length)
throws IOException {
return ((PositionedReadable)in).readFullyScatterGather(position, length);
}
{code}
This allows HDFS to return ByteBuffers all the way to the application. The
above api, in turn, invokes DFSClient.InputStream:
{code}
/**
* Read bytes starting from the specified position. This is optimized
* for fast preads from an application with minimum of buffer copies.
*
* @param position start read from this position
* @param length number of bytes to read
*
* @return A list of Byte Buffers that represent all the data that was
* read from the underlying system.
*/
@Override
public List<ByteBuffer> readFullyScatterGather(long position, int length)
throws IOException {
{code}
Some details here: http://bit.ly/zyDF0h
> ByteBuffer-based read API for DFSInputStream
> --------------------------------------------
>
> Key: HDFS-2834
> URL: https://issues.apache.org/jira/browse/HDFS-2834
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Henry Robinson
> Assignee: Henry Robinson
> Attachments: HDFS-2834-no-common.patch, HDFS-2834.3.patch,
> HDFS-2834.4.patch, HDFS-2834.5.patch, HDFS-2834.6.patch, HDFS-2834.patch,
> HDFS-2834.patch, hdfs-2834-libhdfs-benchmark.png
>
>
> The {{DFSInputStream}} 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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