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https://issues.apache.org/jira/browse/HDFS-1094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12883632#action_12883632
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dhruba borthakur commented on HDFS-1094:
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@Koji: we have files with replication factor of 3. if a large number of
datanodes fail at the same time, we do see missing blocks. Sometimes, the
datanode process on these machines fail to start even after repeated
start-dfs.sh attempts, sometimes the entire machine fails to reboot. Then we
have to manually fix a few of those bad datanode machines and make them come
online; this fixes the "missing blocks" problem but is a manual process and is
painful.
> 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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