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From "Ivan Mitic (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-4677) Editlog should support synchronous writes
Date Sun, 14 Apr 2013 22:54:15 GMT

     [ https://issues.apache.org/jira/browse/HDFS-4677?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Ivan Mitic updated HDFS-4677:
-----------------------------

    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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