[jira] [Created] (HDFS-4677) Editlog should support synchronous writes

["Ivan Mitic (JIRA)" <ji...@apache.org>]

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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