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https://issues.apache.org/jira/browse/HDFS-14111?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16718160#comment-16718160
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Todd Lipcon commented on HDFS-14111:
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Thanks for the links to those other initiatives, Steve. I think, while they'd
be useful improvements, it would be nice to also treat this as a libhdfs bug
and see if we can make an internal change to fix the issue without requiring
callers to move to a new API. It's worth noting that the issue here is with
libhdfs rather than with HDFS itself, as far as I can understand from the code
-- i.e. using the Java open() API _does not_ trigger an eager read from offset
0, but the libhdfs C equivalent _does_.
> 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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