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https://issues.apache.org/jira/browse/HDFS-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13434913#comment-13434913
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Konstantin Shvachko commented on HDFS-3772:
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What problem are you solving?
> HDFS NN will hang in safe mode and never come out if we change the
> dfs.namenode.replication.min bigger.
> -------------------------------------------------------------------------------------------------------
>
> 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 out.
> 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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