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https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ivan Mitic updated HDFS-4677:
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Attachment: HDFS-4677.patch
Attaching the patch. Will also attach the branch-1 compatible patch once this
one is reviewed.
I see that {{EditLogFileOutputStream}} already has a
{{shouldSkipFsyncForTests}} flag which is used to speed up test execution. I
did a quick experiment on my Ubuntu box to see if we can remove this test hook
in favor of the newly introduced config and still keep the test runtime
reasonably low. Turned out that we cannot. TestEditLog which run ~3 minutes
took more than 15 minutes with the new config (it timed out actually), so
keeping the existing logic as-is.
> 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.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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