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https://issues.apache.org/jira/browse/HDFS-15150?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ahmed Hussein updated HDFS-15150:
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Attachment: (was: HDFS-1515-branch-2.10.001.patch)
> Introduce read write lock to Datanode
> -------------------------------------
>
> Key: HDFS-15150
> URL: https://issues.apache.org/jira/browse/HDFS-15150
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 3.3.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Fix For: 3.3.0
>
> Attachments: HDFS-15150.001.patch, HDFS-15150.002.patch,
> HDFS-15150.003.patch
>
>
> HDFS-9668 pointed out the issues around the DN lock being a point of
> contention some time ago, but that Jira went in a direction of creating a new
> FSDataset implementation which is very risky, and activity on the Jira has
> stalled for a few years now. Edit: Looks like HDFS-9668 eventually went in a
> similar direction to what I was thinking, so I will review that Jira in more
> detail to see if this one is necessary.
> I feel there could be significant gains by moving to a ReentrantReadWrite
> lock within the DN. The current implementation is simply a ReentrantLock so
> any locker blocks all others.
> Once place I think a read lock would benefit us significantly, is when the DN
> is serving a lot of small blocks and there are jobs which perform a lot of
> reads. The start of reading any blocks right now takes the lock, but if we
> moved this to a read lock, many reads could do this at the same time.
> The first conservative step, would be to change the current lock and then
> make all accesses to it obtain the write lock. That way, we should keep the
> current behaviour and then we can selectively move some lock accesses to the
> readlock in separate Jiras.
> I would appreciate any thoughts on this, and also if anyone has attempted it
> before and found any blockers.
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