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https://issues.apache.org/jira/browse/HDFS-15?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801009#action_12801009
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eric baldeschwieler commented on HDFS-15:
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Why was it decided that being on two racks is less important than meeting a
replication goal of all replicas?
It seems intuitive to me that making sure that all blocks with 2 replicas be on
two racks should be the higher priority than guaranteeing replicas > 2. In
practice this may not be a big difference, but I don't understand why this
priority was chosen.
> All replicas of a block end up on only 1 rack
> ---------------------------------------------
>
> Key: HDFS-15
> URL: https://issues.apache.org/jira/browse/HDFS-15
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Hairong Kuang
> Assignee: Jitendra Nath Pandey
> Priority: Critical
> Fix For: 0.21.0
>
> Attachments: HDFS-15.4.patch, HDFS-15.5.patch, HDFS-15.6.patch,
> HDFS-15.patch, HDFS-15.patch.2, HDFS-15.patch.3
>
>
> HDFS replicas placement strategy guarantees that the replicas of a block
> exist on at least two racks when its replication factor is greater than one.
> But fsck still reports that the replicas of some blocks end up on one rack.
> The cause of the problem is that decommission and corruption handling only
> check the block's replication factor but not the rack requirement. When an
> over-replicated block loses a replica due to decomission, corruption, or
> heartbeat lost, namenode does not take any action to guarantee that remaining
> replicas are on different racks.
>
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