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From "Colin Patrick McCabe (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3004) Implement Recovery Mode
Date Thu, 05 Apr 2012 23:06:25 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3004?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13247810#comment-13247810
] 

Colin Patrick McCabe commented on HDFS-3004:
--------------------------------------------

An example of recovery in action:

{code}
ERROR namenode.FSImage (FSEditLogLoader.java:loadEditRecords(174)) - Error replaying  edit
log at offset 202.  Expected transaction ID was 5
Recent opcode offsets: 17 74 136 202

ERROR namenode.FSImage (FSEditLogLoader.java:askOperator(111)) - We failed to read    txId
5

namenode.MetaRecoveryContext (MetaRecoveryContext.java:ask(61)) - Enter 'c' to  continue,
skipping the bad section in the log
Enter 's' to stop reading the edit log here, abandoning any later edits
Enter 'q' to quit without saving
Enter 'a' to always select the first choice in the future without prompting(c/s/q/a) c

Continuing.
{code}
                
> 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__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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