Manoj Govindassamy created HDFS-11790:
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Summary: 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
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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