[jira] [Commented] (HDFS-15160) ReplicaMap, Disk Balancer, Directory Scanner and various FsDatasetImpl methods should use datanode readlock

["Ahmed Hussein (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-15160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17379406#comment-17379406 ] 

Ahmed Hussein commented on HDFS-15160:
--------------------------------------

I am opening the issue in order to submit PRs backporting the changes to branches 2.10 - 3.x

> ReplicaMap, Disk Balancer, Directory Scanner and various FsDatasetImpl methods should use datanode readlock
> -----------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-15160
>                 URL: https://issues.apache.org/jira/browse/HDFS-15160
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>    Affects Versions: 3.3.0
>            Reporter: Stephen O'Donnell
>            Assignee: Stephen O'Donnell
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 3.3.1, 3.4.0
>
>         Attachments: HDFS-15160-branch-3.3-001.patch, HDFS-15160.001.patch, HDFS-15160.002.patch, HDFS-15160.003.patch, HDFS-15160.004.patch, HDFS-15160.005.patch, HDFS-15160.006.patch, HDFS-15160.007.patch, HDFS-15160.008.patch, HDFS-15160.branch-3-3.001.patch, image-2020-04-10-17-18-08-128.png, image-2020-04-10-17-18-55-938.png
>
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
>
> Now we have HDFS-15150, we can start to move some DN operations to use the read lock rather than the write lock to improve concurrence. The first step is to make the changes to ReplicaMap, as many other methods make calls to it.
> This Jira switches read operations against the volume map to use the readLock rather than the write lock.
> Additionally, some methods make a call to replicaMap.replicas() (eg getBlockReports, getFinalizedBlocks, deepCopyReplica) and only use the result in a read only fashion, so they can also be switched to using a readLock.
> Next is the directory scanner and disk balancer, which only require a read lock.
> Finally (for this Jira) are various "low hanging fruit" items in BlockSender and fsdatasetImpl where is it fairly obvious they only need a read lock.
> For now, I have avoided changing anything which looks too risky, as I think its better to do any larger refactoring or risky changes each in their own Jira.



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