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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11790) Decommissioning of a DataNode after MaintenanceState takes a very long time to complete
Date Wed, 24 May 2017 05:06:05 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16022321#comment-16022321

Ming Ma commented on HDFS-11790:

Thanks [~manojg] for reporting this. Hmm, the existing code should take care of this. Wonder
if it is due to some corner cases where the following functions don't skip maintenance nodes

* BlockManager#createLocatedBlock should skip IN_MAINTENANCE nodes.
* BlockManager#chooseSourceDatanodes should skip MAINTENANCE_NOT_FOR_READ nodes set for IN_MAINTENANCE

> Decommissioning of a DataNode after MaintenanceState takes a very long time to complete
> ---------------------------------------------------------------------------------------
>                 Key: HDFS-11790
>                 URL: https://issues.apache.org/jira/browse/HDFS-11790
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Manoj Govindassamy
>            Assignee: Manoj Govindassamy
>         Attachments: HDFS-11790-test.01.patch
> *Problem:*
> When a DataNode is requested for Decommissioning after it successfully transitioned to
MaintenanceState (HDFS-7877), the decommissioning state transition is stuck for a long time
even for very small number of blocks in the cluster. 
> *Details:*
> * A DataNode DN1 wa requested for MaintenanceState and it successfully transitioned from
ENTERING_MAINTENANCE state IN_MAINTENANCE state as there are sufficient replication for all
its blocks.
> * As DN1 was in maintenance state now, the DataNode process was stopped on DN1. Later
the same DN1 was requested for Decommissioning. 
> * As part of Decommissioning, all the blocks residing in DN1 were requested for re-replicated
to other DataNodes, so that DN1 could transition from ENTERING_DECOMMISSION to DECOMMISSIONED.

> * But, re-replication for few blocks was stuck for a long time. Eventually it got completed.
> * Digging the code and logs, found that the IN_MAINTENANCE DN1 was chosen as a source
datanode for re-replication of few of the blocks. Since DataNode process on DN1 was already
stopped, the re-replication was stuck for a long time.
> * Eventually PendingReplicationMonitor timed out, and those re-replication were re-scheduled
for those timed out blocks. Again, during the re-replication also, the IN_MAINT DN1 was chose
as a source datanode for few of the blocks leading to timeout again. This iteration continued
for few times until all blocks get re-replicated.
> * By design, IN_MAINT datandoes should not be chosen for any read or write operations.

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