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https://issues.apache.org/jira/browse/HDFS-8992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14721016#comment-14721016
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He Xiaoqiao commented on HDFS-8992:
-----------------------------------
Thanks Tsz Wo Nicholas Sze for your comments, there is only one line of stack
trace:
{code}
2015-06-29 14:06:35,280 WARN org.apache.hadoop.hdfs.server.balancer.Balancer:
Dispatcher thread failed
java.lang.NullPointerException
{code}
> Balancer throws NPE
> -------------------
>
> Key: HDFS-8992
> URL: https://issues.apache.org/jira/browse/HDFS-8992
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: balancer & mover
> Affects Versions: 2.4.1
> Reporter: He Xiaoqiao
>
> Balancer may throw NPE because the following {{if Statements}}
> {code:java|firstline=705}
> synchronized (block) {
> // update locations
> for (String datanodeUuid : blk.getDatanodeUuids()) {
> final BalancerDatanode d = datanodeMap.get(datanodeUuid);
> if (datanode != null) { // not an unknown datanode
> block.addLocation(d);
> }
> }
> }
> {code}
> Before moving block, Balancer divides into two step to get all DNs info &
> some blocks of DN from NN. Regarding one DN commission after
> {{getDatanodeReport}} then {{getBlockList}} and one block's target is above
> DN just right, It will throw NPE when dispatcher.
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