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From Anty <anty....@gmail.com>
Subject Re: Probably a bug in FsEditLog
Date Mon, 08 Apr 2013 09:14:10 GMT
@Harsh
I'm using CDH3u4.However, the processing logical with regard to FSEditlog
between CDH3U4 and hadoop 1.0.2 is almost the same.
So i'm not sure it is proper to file a JIRA?

On Mon, Apr 8, 2013 at 4:33 PM, Harsh J <harsh@cloudera.com> wrote:

> Thanks for analyzing and reporting this Anty,
>
> What version of Apache Hadoop 1.x are you encountering this on? If
> you've spotted the code issue on branch-1, please do log a HDFS JIRA
> with some NN logs and your other details.
>
> On Sun, Apr 7, 2013 at 3:18 PM, Anty <anty.rao@gmail.com> wrote:
> > Hi:ALL
> >
> > In our cluster, we configure the NameNode to write to both local  and NFS
> > mounted directories. When the NFS mounted directory is inaccessible, the
> > NameNode should keep running without error, but our namenode crash with
> > following stack trace.
> >
> > 2013-04-02 23:35:21,535 WARN
> org.apache.hadoop.hdfs.server.common.Storage:
> >> Removing storage dir /nfs2-mount/onest3/dfs/name
> >> 2013-04-02 23:35:21,536 FATAL
> >> org.apache.hadoop.hdfs.server.namenode.FSNamesystem: Unable to find
> edits
> >> stream with IO error
> >> java.lang.Exception: Unable to find edits stream with IO error
> >>         at
> >>
> org.apache.hadoop.hdfs.server.namenode.FSEditLog.fatalExit(FSEditLog.java:430)
> >>         at
> >>
> org.apache.hadoop.hdfs.server.namenode.FSEditLog.removeEditsStreamsAndStorageDirs(FSEditLog.java:519)
> >>         at
> >>
> org.apache.hadoop.hdfs.server.namenode.FSEditLog.logSync(FSEditLog.java:1139)
> >>         at
> >>
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.completeFile(FSNamesystem.java:1641)
> >>         at
> >>
> org.apache.hadoop.hdfs.server.namenode.NameNode.complete(NameNode.java:689)
> >>         at sun.reflect.GeneratedMethodAccessor21.invoke(Unknown Source)
> >>         at
> >>
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> >>         at java.lang.reflect.Method.invoke(Method.java:597)
> >>         at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:557)
> >>         at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1434)
> >>         at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1430)
> >>         at java.security.AccessController.doPrivileged(Native Method)
> >>         at javax.security.auth.Subject.doAs(Subject.java:396)
> >>         at
> >>
> org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1177)
> >>         at org.apache.hadoop.ipc.Server$Handler.run(Server.java:1428)
> >> 2013-04-02 23:35:21,539 INFO
> >> org.apache.hadoop.hdfs.server.namenode.NameNode: SHUTDOWN_MSG:
> >>
> >
> > According to the stack trace, When NameNode tries to sync edit log, it
> does
> > identify the mounted NFS directory is inaccessible, and attempt to remove
> > it from the FSEditLog#editStreams. However, it found the edit stream
> > corresponding to the mounted NFS has already been removed. Under this
> > circumstance, NameNode just kill itself, aborted!
> >
> >  After looking through the source code of HDFS, I found there is another
> > code path of removing edit stream from FSEditLog#editStreams, which can
> > cause above race condition. In method FSEditLog#logEdit
> >
> >
> >>    if (getNumEditStreams() < 1)
> >>         {
> >>             throw new AssertionError("No edit streams to log to");
> >>         }
> >>         long start = FSNamesystem.now();
> >>         for (int idx = 0; idx < editStreams.size(); idx++)
> >>         {
> >>             EditLogOutputStream eStream = editStreams.get(idx);
> >>             try
> >>             {
> >>                 eStream.write(op, writables);
> >>             }
> >>             catch (IOException ioe)
> >>             {
> >>                 removeEditsAndStorageDir(idx);
> >>                 idx--;
> >>             }
> >>         }
> >>
> >>
> > The cause of this race condition lie in FSEditLog#logSync method, there
> are
> > two steps in FSEditLog#logSync
> >
> > 1.    Do sync operation, if one edit stream is accessible, put it into
> > error stream list.(un-synchronized)
> > 2.    Delete error stream from FSEditLog#editStreams(synchronized)
> >
> > Step #1 isn’t synchronized, so there is a possibility that after step#1
> and
> > before step #2 the error stream has already been removed from other
> thread
> > by invoking FSEditLog#logEdit
> >
> > If this was exactly a bug, my proposal fix is in method FSEditlog#logsync
> > ignore or print some warning message instead of aborting the namenode
> when
> > error edit stream doesn't exist in FSEditLog#editStreams
> >
> >
> >
> > --
> > Best Regards
> > Anty Rao
>
>
>
> --
> Harsh J
>



-- 
Best Regards
Anty Rao

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