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https://issues.apache.org/jira/browse/HDFS-6791?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ming Ma updated HDFS-6791:
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Attachment: HDFS-6791-2.patch
Thanks, Jing.
Here is the updated patch that fixed the unused variable issue.
For the rare condition where the decommission might have been completed by the
time stopDataNode is called, the patch set
DFS_DATANODE_BALANCE_BANDWIDTHPERSEC_KEY to 1 so it is going to longer time for
the decommission to complete before the test time outs. Will that work?
> A block could remain under replicated if all of its replicas are on
> decommissioned nodes
> ----------------------------------------------------------------------------------------
>
> Key: HDFS-6791
> URL: https://issues.apache.org/jira/browse/HDFS-6791
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Ming Ma
> Assignee: Ming Ma
> Attachments: HDFS-6791-2.patch, HDFS-6791.patch
>
>
> Here is the scenario.
> 1. Normally before NN transitions a DN to decommissioned state, enough
> replicas have been copied to other "in service" DNs. However, in some rare
> situations, the cluster got into a state where a DN is in decommissioned
> state and a block's only replica is on that DN. In such state, the number of
> replication reported by fsck is 1; the block just stays in under replicated
> state; applications can still read the data, given decommissioned node can
> served read traffic.
> This can happen in some error situations such DN failure or NN failover. For
> example
> a) a block's only replica is node A temporarily.
> b) Start decommission process on node A.
> c) When node A is in "decommission-in-progress" state, node A crashed. NN
> will mark node A as dead.
> d) After node A rejoins the cluster, NN will mark node A as decommissioned.
> 2. In theory, NN should take care of under replicated blocks. But it doesn't
> for this special case where the only replica is on decommissioned node. That
> is because NN has the policy of "decommissioned node can't be picked the
> source node for replication".
> {noformat}
> BlockManager.java
> chooseSourceDatanode
> // never use already decommissioned nodes
> if(node.isDecommissioned())
> continue;
> {noformat}
> 3. Given NN marks the node as decommissioned, admins will shutdown the
> datanode. Under replicated blocks turn into missing blocks.
> 4. The workaround is to recommission the node so that NN can start the
> replication from the node.
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