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Date: Thu, 31 Mar 2011 21:21:06 +0000 (UTC)
From: "Scott Chen (JIRA)" <jira@apache.org>
To: mapreduce-dev@hadoop.apache.org
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Subject: [jira] [Resolved] (MAPREDUCE-1861) Raid should rearrange the
 replicas while raiding
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     [ https://issues.apache.org/jira/browse/MAPREDUCE-1861?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Scott Chen resolved MAPREDUCE-1861.
-----------------------------------

    Resolution: Won't Fix

We found that this approach puts some load on namenode. We will go with the approach in MAPREDUCE-2275.

> 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-v4.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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