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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1094) Intelligent block placement policy to decrease probability of block loss
Date Tue, 13 Apr 2010 22:47:53 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12856673#action_12856673
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Konstantin Shvachko commented on HDFS-1094:
-------------------------------------------

> P(of not losing a specific block) = 1 - 1/C(N,r)

Is that right?
I think you are throwing one replica at a time on the cluster. The probability of first missing
the failed nodes is (N-r)/N. The probability of the second falling into live node excluding
the one that already has the first replica is (N-r-1)/N. And so on.

It would be good if somebody could combine calculations in a pdf (or html) doc and attach
to this jira. We can link it from FAQ then.

> 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: dhruba borthakur
>
> 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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