Todd Lipcon created HDFS-14111:
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Summary: 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
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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