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https://issues.apache.org/jira/browse/HDFS-1432?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12917284#action_12917284
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Sriram Rao commented on HDFS-1432:
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> Performance: Map-reduce jobs could have a performance impact if the number of
> replicas are reduced from 3 to 2. So, the >tradeoff is reducing the total
> amount of storage while possibly increasing job latencies.
With 2 copies in 2 racks, you are still preserving rack locality. That maybe
sufficient.
> HDFS across data centers: HighTide
> ----------------------------------
>
> Key: HDFS-1432
> URL: https://issues.apache.org/jira/browse/HDFS-1432
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> There are many instances when the same piece of data resides on multiple HDFS
> clusters in different data centers. The primary reason being that the
> physical limitation of one data center is insufficient to host the entire
> data set. In that case, the administrator(s) typically partition that data
> into two (or more) HDFS clusters on two different data centers and then
> duplicates some subset of that data into both the HDFS clusters.
> In such a situation, there will be six physical copies of data that is
> duplicated, three copies in one data center and another three copies in
> another data center. It would be nice if we can keep fewer than 3 replicas on
> each of the data centers and have the ability to fix a replica in the local
> data center by copying data from the remote copy in the remote data center.
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