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https://issues.apache.org/jira/browse/HDFS-8647?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14790695#comment-14790695
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Brahma Reddy Battula commented on HDFS-8647:
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{quote}Upgrading from 2.7 to 2.8 (assume we put this change in 2.8) will break
that unless they recompile their custom class. That is because BlockManager
will start calling {{chooseReplicasToDelete}} method which isn't implemented in
their class.{quote}
AFAIK most customized policy extends BlockPlacementPolicyDefault. It's rare to
implement a policy from scratch. Not only method signature, as long as we
change default policy one byte, the customized policy need recompile anyway.
Maven can solve the dependency and recompile if needed.
The old {{chooseReplicasToDelete}} is limited. I suppose little user override
it. After the change, it's more flexible to let user customize it. If user
didn't override it before, then the customized policy class can cleanly
recompile without complain.
{quote}Note that this seems similar to BlockPlacementPolicy's
verifyBlockPlacement. So if they can be combined, that is nice.{quote}
That's a good idea.
> Abstract BlockManager's rack policy into BlockPlacementPolicy
> -------------------------------------------------------------
>
> Key: HDFS-8647
> URL: https://issues.apache.org/jira/browse/HDFS-8647
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Ming Ma
> Assignee: Brahma Reddy Battula
> Attachments: HDFS-8647-001.patch, HDFS-8647-002.patch,
> HDFS-8647-003.patch
>
>
> Sometimes we want to have namenode use alternative block placement policy
> such as upgrade domains in HDFS-7541.
> BlockManager has built-in assumption about rack policy in functions such as
> useDelHint, blockHasEnoughRacks. That means when we have new block placement
> policy, we need to modify BlockManager to account for the new policy. Ideally
> BlockManager should ask BlockPlacementPolicy object instead. That will allow
> us to provide new BlockPlacementPolicy without changing BlockManager.
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