Ivan Mitic created HDFS-4677:
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Summary: Editlog should support synchronous writes
Key: HDFS-4677
URL: https://issues.apache.org/jira/browse/HDFS-4677
Project: Hadoop HDFS
Issue Type: Bug
Affects Versions: 3.0.0, 1-win
Reporter: Ivan Mitic
Assignee: Ivan Mitic
In the current implementation, NameNode editlog performs syncs to the
persistent storage using the {{FileChannel#force}} Java APIs. This API is
documented to be slower compared to an alternative where {{RandomAccessFile}}
is opened with "rws" flags (synchronous writes).
We instrumented {{FileChannel#force}} on Windows and it some software/hardware
configurations it can perform significantly slower than the “rws” alternative.
In terms of the Windows APIs, FileChannel#force internally calls
[FlushFileBuffers|http://msdn.microsoft.com/en-us/library/windows/desktop/aa364439(v=vs.85).aspx]
while RandomAccessFile (“rws”) opens the file with the
[FILE_FLAG_WRITE_THROUGH flag|http://support.microsoft.com/kb/99794].
With this Jira I'd like to introduce a flag that provide means to configure
NameNode to use synchronous writes. There is a catch though, the behavior of
the "rws" flags is platform and hardware specific and might not provide the
same level of guarantees as {{FileChannel#force}} w.r.t. flushing the on-disk
cache. This is an expert level setting, and it should be documented as such.
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