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https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13626712#comment-13626712
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Daryn Sharp commented on HDFS-4677:
-----------------------------------
A bit dated, but interesting data:
http://stas-blogspot.blogspot.com/2011/11/java-file-flushing-performance.html
> 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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