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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4677) Editlog should support synchronous writes
Date Sun, 09 Jun 2013 00:23:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13678895#comment-13678895

Hadoop QA commented on HDFS-4677:

{color:red}-1 overall{color}.  Here are the results of testing the latest attachment 
  against trunk revision .

    {color:red}-1 patch{color}.  The patch command could not apply the patch.

Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/4501//console

This message is automatically generated.
> 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
>         Attachments: HDFS-4677.2.patch, HDFS-4677.3.patch, HDFS-4677.3.patch, HDFS-4677.branch-1-win.2.patch,
HDFS-4677.branch-1-win.patch, HDFS-4677.patch
> 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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