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From "Scott Chen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1861) Raid should rearrange the replicas while raiding
Date Thu, 09 Dec 2010 22:39:02 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12969981#action_12969981
] 

Scott Chen commented on MAPREDUCE-1861:
---------------------------------------

{code}
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     [exec] +1 overall.
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     [exec]     +1 @author.  The patch does not contain any @author tags.
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     [exec]     +1 tests included.  The patch appears to include 3 new or modified tests.
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     [exec]     +1 javadoc.  The javadoc tool did not generate any warning messages.
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     [exec]     +1 javac.  The applied patch does not increase the total number of javac compiler
warnings.
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     [exec]     +1 findbugs.  The patch does not introduce any new Findbugs (version 1.3.9)
warnings.
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     [exec]     +1 release audit.  The applied patch does not increase the total number of
release audit warnings.
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     [exec]     +1 system test framework.  The patch passed system test framework compile.
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     [exec] ======================================================================
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     [exec]     Finished build.
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{code}

> 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: 0.23.0
>            Reporter: Scott Chen
>            Assignee: Scott Chen
>             Fix For: 0.23.0
>
>         Attachments: MAPREDUCE-1861-v2.txt, MAPREDUCE-1861-v3.txt, MAPREDUCE-1861.txt,
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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