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https://issues.apache.org/jira/browse/HDFS-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13431115#comment-13431115
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Yanbo Liang commented on HDFS-3772:
-----------------------------------
After survey the source code, I found that after modifying the minimum
replication, the HDFS persistent storage has no information about the former
minimum replication. So we can not get the it after restart. So we need to
think out of box.
I have found the blockThreshold(dfs.namenode.safemode.threshold-pct) specifies
the percentage of blocks that should satisfy the minimal replication
requirement defined by dfs.namenode.replication.min. I think we can change the
sementics of this parameter to the percentage of blocks that satisfy the real
replication of each file. And then we compare the NN received replication with
the real replication of each file. If they are equal, we increment blockSafe.
Anyone who have some opinions? If this scenario is ok, I will fix it.
> 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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