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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1094) Intelligent block placement policy to decrease probability of block loss
Date Wed, 07 Jul 2010 06:30:56 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12885848#action_12885848
] 

Hairong Kuang commented on HDFS-1094:
-------------------------------------

Rodrigo, thanks for your explanation. Now I understand your proposal much better.

> We assume machine failures are independent. 

I am not sure if I agree with this assumption. From the attached analysis, the in-rack placement
has the lowest data loss probability. This is counter-intuitive. in reality, the chance of
losing a rack is not small. So a block placement policy normally place a block in at least
two racks. 

> 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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