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https://issues.apache.org/jira/browse/HDFS-4702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13632525#comment-13632525
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Chris Nauroth commented on HDFS-4702:
-------------------------------------
The lock happens in {{DatanodeManager#fetchDatanodes}}, but it's not
immediately obvious to me why we need the namesystem lock here. Perhaps it's
related to the edit log ops {{OP_DATANODE_ADD}} and {{OP_DATANODE_REMOVE}},
which are deprecated and now appear to be unused? Maybe it's safe to remove
the lock now, but this will need more investigation.
> remove namesystem lock from DatanodeManager#fetchDatanodes
> ----------------------------------------------------------
>
> Key: HDFS-4702
> URL: https://issues.apache.org/jira/browse/HDFS-4702
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Affects Versions: 3.0.0
> Reporter: Chris Nauroth
>
> {{DatanodeManager#fetchDatanodes}} currently holds the namesystem read lock
> while iterating through data nodes. This method is called from the namenode
> web UI. HDFS-3990 reported a performance problem in this code path. This is
> a follow-up jira to investigate whether or not we can remove the lock.
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