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Date: Tue, 4 Sep 2007 19:42:45 -0700 (PDT)
From: "Konstantin Shvachko (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (HADOOP-1076) Periodic checkpointing cannot
 resume if the secondary name-node fails.
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    [ https://issues.apache.org/jira/browse/HADOOP-1076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12524955 ] 

Konstantin Shvachko commented on HADOOP-1076:
---------------------------------------------

I managed to corrupt current name-node image using your patch.
Actually the image was set to an empty file, so that the name-node would not even restart after the checkpoint.
I started 2 secondary nodes. The first of them was in the middle of getFSImage(), when the second called rollFSImage()
and received the following exception:

java.lang.IllegalStateException: Committed
	at org.mortbay.jetty.servlet.ServletHttpResponse.resetBuffer(ServletHttpResponse.java:212)
	at org.mortbay.jetty.servlet.ServletHttpResponse.sendError(ServletHttpResponse.java:375)
	at org.apache.hadoop.dfs.SecondaryNameNode$GetImageServlet.doGet(SecondaryNameNode.java:455)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:427)
	at org.mortbay.jetty.servlet.WebApplicationHandler.dispatch(WebApplicationHandler.java:475)
	at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:567)
	at org.mortbay.http.HttpContext.handle(HttpContext.java:1565)
	at org.mortbay.jetty.servlet.WebApplicationContext.handle(WebApplicationContext.java:635)
	at org.mortbay.http.HttpContext.handle(HttpContext.java:1517)
	at org.mortbay.http.HttpServer.service(HttpServer.java:954)
	at org.mortbay.http.HttpConnection.service(HttpConnection.java:814)
	at org.mortbay.http.HttpConnection.handleNext(HttpConnection.java:981)
	at org.mortbay.http.HttpConnection.handle(HttpConnection.java:831)
	at org.mortbay.http.SocketListener.handleConnection(SocketListener.java:244)
	at org.mortbay.util.ThreadedServer.handle(ThreadedServer.java:357)
	at org.mortbay.util.ThreadPool$PoolThread.run(ThreadPool.java:534)

This is likely to be related to the patch, since the second secondary node would just get an exception trying to rollEditsLog().
My guess is that your patch prohibits to rollFSImage() if the edits log was not rolled, instead of
prohibiting 2 simultaneous rollFSImage().


> Periodic checkpointing cannot resume if the secondary name-node fails.
> ----------------------------------------------------------------------
>
>                 Key: HADOOP-1076
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1076
>             Project: Hadoop
>          Issue Type: Bug
>          Components: dfs
>            Reporter: Konstantin Shvachko
>            Assignee: dhruba borthakur
>             Fix For: 0.15.0
>
>         Attachments: secondaryRestart.patch
>
>
> If secondary name-node fails during checkpointing then the primary node will have 2 edits file.
> "edits" - is the one which current checkpoint is to be based upon.
> "edits.new" - is where new name space edits are currently logged.
> The problem is that the primary node cannot do checkpointing until "edits.new" file is in place.
> That is, even if the secondary name-node is restarted periodic checkpointing is not going to be resumed.
> In fact the primary node will be throwing an exception complaining about the existing "edits.new"
> There is only one way to get rid of the edits.new file - to restart the primary name-node.
> So in a way if secondary name-node fails then you should restart the whole cluster.
> Here is a rather simple modification to the current approach, which we discussed with Dhruba.
> When secondary node requests to rollEditLog() the primary node should roll the edit log only if
> it has not been already rolled. Otherwise the existing "edits" file will be used for checkpointing
> and the primary node will keep accumulating new edits in the "edits.new".
> In order to make it work the primary node should also ignore any rollFSImage() requests when it
> already started to perform one. Otherwise the new image can become corrupted if two secondary
> nodes request to rollFSImage() at the same time.
> 2. Also, after the periodic checkpointing patch HADOOP-227 I see pieces of unusable code.
> I noticed one data member SecondaryNameNode.localName and at least 4 methods in FSEditLog
> that are not used anywhere. We should remove them and others alike if found.
> Supporting unusable code is such a waist of time.

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