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https://issues.apache.org/jira/browse/HDFS-14111?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16783841#comment-16783841
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Sahil Takiar commented on HDFS-14111:
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I think this patch should be good to merge.
Talked to a few other folks about {{errno}} handling, and given that the C docs
say {{The value in errno is significant only when the return value of the call
indicated an error (i.e., -1 from most system calls; -1 or NULL from most
library functions); a function that succeeds is allowed to change errno.}} I
think the test changes made as part of this patch should be fine.
> 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
> Assignee: Sahil Takiar
> Priority: Major
> Attachments: HDFS-14111.001.patch, HDFS-14111.002.patch,
> HDFS-14111.003.patch
>
>
> 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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