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https://issues.apache.org/jira/browse/HDFS-2184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13069606#comment-13069606
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Suresh Srinivas commented on HDFS-2184:
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There is some work Nicholas is also doing separating block management out of
namenode and move locks related to block management out of FSNamesystem.
> Revisit Namenode locking
> ------------------------
>
> Key: HDFS-2184
> URL: https://issues.apache.org/jira/browse/HDFS-2184
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: Eli Collins
>
> While working on HDFS-988 I noticed that the locking in FSNamesystem and
> FSDirectory could be improved. Some observations:
> The namesystem lock (fsLock) is always taken before acquiring the directory
> lock (dirLock). Therefore the only time when the directory lock is needed is
> when the fsLock is taken for reading and the directory lock is taken for
> writing, but I don't think that ever happens. Therefore we can probably get
> rid of the directory lock.
> In HDFS-988 I modified handleHeartbeat to take the read lock so it's
> synchronized with register datanode. I also added a missing synchronization
> of datanodeMap to wipeDatanode because handleHeartbeat calls getDatanode()
> while only holding locks on heartbeats and datanodeMap, but registerDatanode
> mutates datanodeMap without locking either. We should revisit which
> locks/synchronization protect which data structures, there may be other
> similar bugs and also opportunities to increase parallelism.
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