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https://issues.apache.org/jira/browse/HDFS-1094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12885849#action_12885849
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Rodrigo Schmidt commented on HDFS-1094:
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The assumption that failures are independent was made to ease the calculations.
Assuming that failures are dependent would add too much complexity to the
problem.
In the implementation, we are still trying to keep blocks on separate racks and
that's why I said there will be two windows: one for the remote rack to be
chosen, and the other to limit the machines that can be chosen inside that rack
(for each block that must be replicated).
> Intelligent block placement policy to decrease probability of block loss
> ------------------------------------------------------------------------
>
> Key: HDFS-1094
> URL: https://issues.apache.org/jira/browse/HDFS-1094
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: dhruba borthakur
> Assignee: Rodrigo Schmidt
> Attachments: prob.pdf, prob.pdf
>
>
> The current HDFS implementation specifies that the first replica is local and
> the other two replicas are on any two random nodes on a random remote rack.
> This means that if any three datanodes die together, then there is a
> non-trivial probability of losing at least one block in the cluster. This
> JIRA is to discuss if there is a better algorithm that can lower probability
> of losing a block.
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