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From "Colin Patrick McCabe (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3004) Implement Recovery Mode
Date Sat, 07 Apr 2012 01:09:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3004?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13249109#comment-13249109
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Colin Patrick McCabe commented on HDFS-3004:
--------------------------------------------

Hi Allen,

Recovery uses the normal edit log loading code, so we support loading slightly older versions,
just the same way that the normal edit log loading code does.  The EditLogInputStream will
give you back correctly deserialized opCodes for several different version flavors.  EditLogInputStream::getVersion
will let you know what version of the edit log you are reading.

If your version is too old, you'll have to upgrade it with "./bin/hadoop namenode -upgrade".
 Unfortunately, we don't support combining recovery mode and upgrade mode.  We could potentially
address this in the future if there is enough demand, but I'm not sure there will be.
                
> Implement Recovery Mode
> -----------------------
>
>                 Key: HDFS-3004
>                 URL: https://issues.apache.org/jira/browse/HDFS-3004
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: tools
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>         Attachments: HDFS-3004.010.patch, HDFS-3004.011.patch, HDFS-3004.012.patch, HDFS-3004.013.patch,
HDFS-3004.015.patch, HDFS-3004.016.patch, HDFS-3004.017.patch, HDFS-3004.018.patch, HDFS-3004.019.patch,
HDFS-3004.020.patch, HDFS-3004.022.patch, HDFS-3004.023.patch, HDFS-3004.024.patch, HDFS-3004.026.patch,
HDFS-3004.027.patch, HDFS-3004.029.patch, HDFS-3004.030.patch, HDFS-3004.031.patch, HDFS-3004.032.patch,
HDFS-3004.033.patch, HDFS-3004.034.patch, HDFS-3004.035.patch, HDFS-3004.036.patch, HDFS-3004.037.patch,
HDFS-3004.038.patch, HDFS-3004.039.patch, HDFS-3004.040.patch, HDFS-3004.041.patch, HDFS-3004.042.patch,
HDFS-3004.042.patch, HDFS-3004.042.patch, HDFS-3004__namenode_recovery_tool.txt
>
>
> When the NameNode metadata is corrupt for some reason, we want to be able to fix it.
 Obviously, we would prefer never to get in this case.  In a perfect world, we never would.
 However, bad data on disk can happen from time to time, because of hardware errors or misconfigurations.
 In the past we have had to correct it manually, which is time-consuming and which can result
in downtime.
> Recovery mode is initialized by the system administrator.  When the NameNode starts up
in Recovery Mode, it will try to load the FSImage file, apply all the edits from the edits
log, and then write out a new image.  Then it will shut down.
> Unlike in the normal startup process, the recovery mode startup process will be interactive.
 When the NameNode finds something that is inconsistent, it will prompt the operator as to
what it should do.   The operator can also choose to take the first option for all prompts
by starting up with the '-f' flag, or typing 'a' at one of the prompts.
> I have reused as much code as possible from the NameNode in this tool.  Hopefully, the
effort that was spent developing this will also make the NameNode editLog and image processing
even more robust than it already is.

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