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Date: Wed, 1 Aug 2012 16:05:06 +0000 (UTC)
From: "Hadoop QA (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (HBASE-6435) Reading WAL files after a recovery
 leads to time lost in HDFS timeouts when using dead datanodes
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    [ https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13426714#comment-13426714 ] 

Hadoop QA commented on HBASE-6435:
----------------------------------

-1 overall.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12538789/6435.v9.patch
  against trunk revision .

    +1 @author.  The patch does not contain any @author tags.

    +1 tests included.  The patch appears to include 8 new or modified tests.

    +1 hadoop2.0.  The patch compiles against the hadoop 2.0 profile.

    +1 javadoc.  The javadoc tool did not generate any warning messages.

    -1 javac.  The applied patch generated 5 javac compiler warnings (more than the trunk's current 4 warnings).

    -1 findbugs.  The patch appears to introduce 6 new Findbugs (version 1.3.9) warnings.

    +1 release audit.  The applied patch does not increase the total number of release audit warnings.

     -1 core tests.  The patch failed these unit tests:
                       org.apache.hadoop.hbase.client.TestAdmin
                  org.apache.hadoop.hbase.fs.TestBlockReorder

Test results: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//testReport/
Findbugs warnings: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//artifact/trunk/patchprocess/newPatchFindbugsWarningshbase-hadoop2-compat.html
Findbugs warnings: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//artifact/trunk/patchprocess/newPatchFindbugsWarningshbase-server.html
Findbugs warnings: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//artifact/trunk/patchprocess/newPatchFindbugsWarningshbase-hadoop1-compat.html
Findbugs warnings: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//artifact/trunk/patchprocess/newPatchFindbugsWarningshbase-common.html
Findbugs warnings: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//artifact/trunk/patchprocess/newPatchFindbugsWarningshbase-hadoop-compat.html
Console output: https://builds.apache.org/job/PreCommit-HBASE-Build/2472//console

This message is automatically generated.
                
> Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead datanodes
> ------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-6435
>                 URL: https://issues.apache.org/jira/browse/HBASE-6435
>             Project: HBase
>          Issue Type: Improvement
>          Components: master, regionserver
>    Affects Versions: 0.96.0
>            Reporter: nkeywal
>            Assignee: nkeywal
>         Attachments: 6435.unfinished.patch, 6435.v2.patch, 6435.v7.patch, 6435.v8.patch, 6435.v9.patch, 6435.v9.patch
>
>
> HBase writes a Write-Ahead-Log to revover from hardware failure.
> This log is written with 'append' on hdfs.
> Through ZooKeeper, HBase gets informed usually in 30s that it should start the recovery process. 
> This means reading the Write-Ahead-Log to replay the edits on the other servers.
> In standards deployments, HBase process (regionserver) are deployed on the same box as the datanodes.
> It means that when the box stops, we've actually lost one of the edits, as we lost both the regionserver and the datanode.
> As HDFS marks a node as dead after ~10 minutes, it appears as available when we try to read the blocks to recover. As such, we are delaying the recovery process by 60 seconds as the read will usually fail with a socket timeout. If the file is still opened for writing, it adds an extra 20s + a risk of losing edits if we connect with ipc to the dead DN.
> Possible solutions are:
> - shorter dead datanodes detection by the NN. Requires a NN code change.
> - better dead datanodes management in DFSClient. Requires a DFS code change.
> - NN customisation to write the WAL files on another DN instead of the local one.
> - reordering the blocks returned by the NN on the client side to put the blocks on the same DN as the dead RS at the end of the priority queue. Requires a DFS code change or a kind of workaround.
> The solution retained is the last one. Compared to what was discussed on the mailing list, the proposed patch will not modify HDFS source code but adds a proxy. This for two reasons:
> - Some HDFS functions managing block orders are static (MD5MD5CRC32FileChecksum). Implementing the hook in the DFSClient would require to implement partially the fix, change the DFS interface to make this function non static, or put the hook static. None of these solution is very clean. 
> - Adding a proxy allows to put all the code in HBase, simplifying dependency management.
> Nevertheless, it would be better to have this in HDFS. But this solution allows to target the last version only, and this could allow minimal interface changes such as non static methods.
> Moreover, writing the blocks to the non local DN would be an even better solution long term.

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