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From "Ivan Mitic (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4677) Editlog should support synchronous writes
Date Tue, 09 Apr 2013 23:40:17 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13627273#comment-13627273
] 

Ivan Mitic commented on HDFS-4677:
----------------------------------

bq. http://stas-blogspot.blogspot.com/2011/11/java-file-flushing-performance.html
Thanks, indeed interesting data. Diff on Windows between "rwd" (which is also based on FILE_FLAG_WRITE_THROUGH)
and {{FileChannel#force}} looks reasonable.
                
> 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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