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https://issues.apache.org/jira/browse/MAPREDUCE-1861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12886816#action_12886816
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Scott Chen commented on MAPREDUCE-1861:
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Thanks for the review, Rodrigo.
I am resubmitting this to Hudson because there has been no response for a while.
> Raid should rearrange the replicas while raiding
> ------------------------------------------------
>
> Key: MAPREDUCE-1861
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1861
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: contrib/raid
> Affects Versions: 0.22.0
> Reporter: Scott Chen
> Assignee: Scott Chen
> Fix For: 0.22.0
>
> Attachments: MAPREDUCE-1861.txt
>
>
> Raided file introduce extra dependencies on the blocks on the same stripe.
> Therefore we need a new way to place the blocks.
> It is desirable that raided file satisfies the following two conditions:
> a. Replicas on the same stripe should be on different machines (or racks)
> b. Replicas of the same block should be on different racks
> MAPREDUCE-1831 will try to delete the replicas on the same stripe and the
> same machine (a).
> But in the mean time, it will try to maintain the number of distinct racks of
> one block (b).
> We cannot satisfy (a) and (b) at the same time with the current logic in
> BlockPlacementPolicyDefault.chooseTarget().
> One choice we have is to change BlockPlacementPolicyDefault.chooseTarget().
> However, this placement is in general good for all files including the
> unraided ones.
> It is not clear to us that we can make this good for both raided and unraided
> files.
> So we propose this idea that when raiding the file. We create one more
> off-rack replica (so the replication=4 now).
> Than we delete two blocks using the policy in MAPREDUCE-1831 after that
> (replication=2 now).
> This way we can rearrange the replicas to satisfy (a) and (b) at the same
> time.
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