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https://issues.apache.org/jira/browse/HDFS-15?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12745196#action_12745196
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Jitendra Nath Pandey commented on HDFS-15:
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Updated proposal:
A different queue (neededReplicationsForRacks) is maintained for blocks
which do not have sufficient racks assigned. Blocks in this queue are treated
with lower priority than the blocks in neededReplications, therefore the
priority logic is same as in comment#3. The reasons for this change:
1. The semantics of neededReplications queue remains unchanged and it
contains only those blocks which are really under-replicated.
2. Keeps the code cleaner as we maintain seperate queue for blocks with not
enough racks. No code change needed to UnderReplicatedBlocks.java. This new
queue can be implemented as just a TreeSet<Block>, therefore no new class needs
to be implemented.
Point # 3 in previous comment remains same except that a block is added to
neededReplicationsForRacks if it doesn't have enough racks.
Point #4 in previous comment remains unchanged.
> 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
>
> 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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