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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3772) HDFS NN will hang in safe mode and never come out if we change the dfs.namenode.replication.min bigger.
Date Wed, 15 Aug 2012 17:12:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13435307#comment-13435307

Konstantin Shvachko commented on HDFS-3772:

It is not the hang. It works as designed. You increase {{replication.min}} and SafeMode waits
for replicas to reach that safe replication limit.
You seem to assume that bumping up {{replication.min}} will result in more replicas, but it
is not the case. You can use setReplication() for that.
> HDFS NN will hang in safe mode and never come out if we change the dfs.namenode.replication.min
> -------------------------------------------------------------------------------------------------------
>                 Key: HDFS-3772
>                 URL: https://issues.apache.org/jira/browse/HDFS-3772
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>    Affects Versions: 2.0.0-alpha
>            Reporter: Yanbo Liang
> If the NN restarts with a new minimum replication (dfs.namenode.replication.min), any
files created with the old replication count will expected to bump up to the new minimum upon
restart automatically. However, the real case is that if the NN restarts will a new minimum
replication which is bigger than the old one, the NN will hang in safemode and never come
> The corresponding test case can pass is because we have missing some test coverage. It
had been discussed in HDFS-3734.
> If the NN received enough number of reported block which is satisfying the new minimum
replication, it will exit safe mode. However, if we change a bigger minimum replication, there
will be no enough amount blocks which are satisfying the limitation.
> Look at the code segment in FSNamesystem.java:
> private synchronized void incrementSafeBlockCount(short replication) {
>       if (replication == safeReplication) {
>         this.blockSafe++;
>         checkMode();
>       }
>     }
> The DNs report blocks to NN and if the replication is equal to safeReplication(It is
assigned by the new minimum replication.), we will increment blockSafe. But if we change a
bigger minimum replication, all the blocks whose replications are lower than it can not satisfy
this equal relationship. But actually the NN had received complete block information. It cause
blockSafe will not increment as usual and not reach the enough amount to exit safe mode and
then NN hangs.

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