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https://issues.apache.org/jira/browse/HDFS-15160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17381522#comment-17381522
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Ahmed Hussein commented on HDFS-15160:
--------------------------------------
Backporting HDFS-15160 to branch-3.2
* Created [PR-3200|https://github.com/apache/hadoop/pull/3200] for branch-3.2
** First: HDFS-15150. Introduce read write lock to Datanode. Contributed
Stephen O'Donnell
`git cherry-pick -x 2a67e2b1a0e3a5f91056f5b977ef9c4c07ba6718`
*** main conflicts:
**** `FsDatasetImpl.java.deepCopyReplica`
**** `FoldedTreeSet` is not used in branch-3.2
*** testing:
```
mvn test -Dtest=TestWriteToReplica,TestReplicaMap,\
TestProvidedImpl,TestFsVolumeList,TestInterDatanodeProtocol
```
** Second: HDFS-15160
`git cherry-pick -x 2a67e2b1a0e3a5f91056f5b977ef9c4c07ba6718`
*** main conflicts:
**** `ReplicaMap.java`
**** `FsDatasetImpl.java.deepCopyReplica`
**** `DirectoryScanner.scan()` conflict in the entire loop.
HDFS-15415. _Reduce locking in Datanode DirectoryScanner_. Which is
already committed to branch-2.10 removes what HDFS-15160 has added. So, I
skipped that.
** Third: HDFS-15457. TestFsDatasetImpl fails intermittently (#2407) and
HDFS-15818. Fix TestFsDatasetImpl.testReadLockCanBeDisabledByConfig (#2679)
`git cherry-pick -x 98097b8f19789605b9697f6a959da57261e0fe19
9434c1eccc255a25ea5e11f6d8c9e1f83996d6b4`
*** main conflicts: Java imports
> 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: 1h 10m
> 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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