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https://issues.apache.org/jira/browse/HDFS-14111?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16769821#comment-16769821
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Sahil Takiar commented on HDFS-14111:
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For Todd's first suggestion. I think something like that already exists -
{{o.a.h.fs.StreamCapabilities}} seems to provide this functionality (someone
correct me if I am wrong). Essentially, it defines what interfaces a stream
implements. Right now it has support for {{Syncable}}, {{CanUnbuffer}},
{{CanSetReadahead}}, and {{CanSetDropBehind}}. We would just need to add
support for {{ByteBufferReadable}} and then libhdfs can call
{{StreamCapabilities#hasCapability}} to determine if the underlying stream
supports {{readDirect}}.
If my approach makes sense, I can start working on a patch.
> hdfsOpenFile on HDFS causes unnecessary IO from file offset 0
> -------------------------------------------------------------
>
> Key: HDFS-14111
> URL: https://issues.apache.org/jira/browse/HDFS-14111
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client, libhdfs
> Affects Versions: 3.2.0
> Reporter: Todd Lipcon
> Priority: Major
>
> hdfsOpenFile() calls readDirect() with a 0-length argument in order to check
> whether the underlying stream supports bytebuffer reads. With DFSInputStream,
> the read(0) isn't short circuited, and results in the DFSClient opening a
> block reader. In the case of a remote block, the block reader will actually
> issue a read of the whole block, causing the datanode to perform unnecessary
> IO and network transfers in order to fill up the client's TCP buffers. This
> causes performance degradation.
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