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https://issues.apache.org/jira/browse/HDFS-7891?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14380482#comment-14380482
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Zhe Zhang commented on HDFS-7891:
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Thanks Nicholas and Walter for the discussion!
bq. I think when we use stripe layout, even when stripe layout is not used. All
6+3 replica(it's better called blocks belonging to same group) comes from
client. As long as the replica is place on the remote rack, the traffic is the
same.
Under the striping layout, in _initial file creation_, all raw or parity data
comes directly from client. So inter-rack traffic won't be affected by
placement policy (unless we consider the factor that client is likely
co-located with 1st DN).
Two other scenarios do incur inter-DN traffic:
# Background block reconstruction. Here a new DN is chosen as reconstruction
destination and will pull data from other DNs in the group
# EC+contiguous layout. Here all blocks are initially triplicated. Then for
each formed group, a new DN is selected as the encoder, which pulls data from
raw data blocks
> 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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