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From "Jitendra Nath Pandey (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1094) Intelligent block placement policy to decrease probability of block loss
Date Wed, 14 Apr 2010 01:11:54 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12856716#action_12856716

Jitendra Nath Pandey commented on HDFS-1094:

> 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.
Shouldn't it be (N-r)/N , (N-r-1)/N-1  , (N-r-2)/N-2 and so on ?
Similarly, probability that all replicas reside on the failed nodes, it would be 
r/N * (r-1)/(N-1) * ... * 1/(N-r+1) = 1/C(N,r)  which is same as in Karthik's formula. 

> 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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