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From "Chris Nauroth (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4677) Editlog should support synchronous writes
Date Wed, 22 May 2013 16:11:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13664217#comment-13664217
] 

Chris Nauroth commented on HDFS-4677:
-------------------------------------

+1 for both trunk and branch-1-win patches.  Thank you for tracking down this performance
issue and contributing a fix!

{quote}
Quick question, should this go to branch-2 as well, given that there was a bit of refactoring
going on? This would make things easier for future backports. I just checked, and the patch
is almost completely compatible.
{quote}

It would be possible to get this into branch-2, considering that the patch doesn't really
have any Windows-specific code.  If we put it in branch-2 right now, then it would be less
code to merge later.

It looks like we'd have to resolve a conflict in {{TestNNStorageRetentionManager}}.  I'd suggest
that if it's easy enough to resolve that conflict now, then we might as well go for it.
                
> 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.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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