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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 Tue, 14 Aug 2012 17:41:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13434293#comment-13434293
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Konstantin Shvachko commented on HDFS-3772:
-------------------------------------------

I was writing this when Jira went down. Will try to reproduce that comment.

> files created with the old replication count will expected to bump up to the new minimum
upon restart automatically

This is not an expected behavior.
{{dfs.namenode.replication.min}} has two purposes:
# Counting the blocks satisfying the new minimum replication during startup.
# Controls the minimal number of replicas that must be created during pipeline in order to
call the data transfer successful.

Setting {{replication.min}} to higher value does not mean NN replicates blocks to that min.

It means NN will wait for that many replicas to be reported during startup before exiting
SafeMode.
If you set it too high, this is one of the ways to never let NN go out of SafeMode automatically.

SafeMode prohibits replication or deletion of blocks or modification of the namespace,
so block replication will not happen until NN leaves SafeMode.
If you are trying to increase block replication for all files in your file system you should
use
{{setReplication()}} on the root recursively. But replication will start only after SafeMode
is OFF.

> I think we can change the semantics of this parameter to the percentage of blocks that
satisfy the real replication of each file.

Not a good idea. In general, changing semantics of existing parameters is confusing. 
And in particular, because this will make NN stay in SafeMode forever if some DataNodes don't
come up.

I think the question here is what you are trying to achieve with this? 
                
> 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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