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https://issues.apache.org/jira/browse/HDFS-7891?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14381864#comment-14381864
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Tsz Wo Nicholas Sze commented on HDFS-7891:
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> ... . But I think place 2 replica under same rack doesn't reduce traffic. ...
It does reduce a remote rack traffic in various cases shown below. Do you
agree?
Keys: LN = local node, LR = local rack, RR = remote rack
|| || 1 replica per rack || 2 replicas per rack ||
| Block group creation | 1 LN + 8 RR | 1 LN + 1 LR + 7 RR |
| Read | 1 LN + 5 RR | 1 LN + 1 LR + 4 RR |
| Reconstruction for 1-missing | 1 LN + 6 RR | 1 LN + 1 LR + 5 RR |
| Reconstruction for 2-missing | 1 LN + 7 RR | 1 LN + 1 LR + 6 RR |
| Reconstruction for 3-missing | 1 LN + 8 RR | 1 LN + 1 LR + 7 RR |
> A block placement policy with best fault tolerance
> --------------------------------------------------
>
> Key: HDFS-7891
> URL: https://issues.apache.org/jira/browse/HDFS-7891
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Walter Su
> Assignee: Walter Su
> Attachments: HDFS-7891.002.patch, HDFS-7891.003.patch,
> HDFS-7891.004.patch, HDFS-7891.patch, PlacementPolicyBenchmark.txt,
> testresult.txt
>
>
> a block placement policy tries its best to place replicas to most racks.
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