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https://issues.apache.org/jira/browse/HDFS-4270?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Derek Dagit updated HDFS-4270:
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Status: Patch Available (was: Open)
> Replications of the highest priority should be allowed to choose a source
> datanode that has reached its max replication limit
> -----------------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-4270
> URL: https://issues.apache.org/jira/browse/HDFS-4270
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Affects Versions: 0.23.5, 3.0.0
> Reporter: Derek Dagit
> Assignee: Derek Dagit
> Priority: Minor
> Attachments: HDFS-4270-branch-0.23.patch, HDFS-4270.patch,
> HDFS-4270.patch
>
>
> Blocks that have been identified as under-replicated are placed on one of
> several priority queues. The highest priority queue is essentially reserved
> for situations in which only one replica of the block exists, meaning it
> should be replicated ASAP.
> The ReplicationMonitor periodically computes replication work, and a call to
> BlockManager#chooseUnderReplicatedBlocks selects a given number of
> under-replicated blocks, choosing blocks from the highest-priority queue
> first and working down to the lowest priority queue.
> In the subsequent call to BlockManager#computeReplicationWorkForBlocks, a
> source for the replication is chosen from among datanodes that have an
> available copy of the block needed. This is done in
> BlockManager#chooseSourceDatanode.
> chooseSourceDatanode's job is to choose the datanode for replication. It
> chooses a random datanode from the available datanodes that has not reached
> its replication limit (preferring datanodes that are currently
> decommissioning).
> However, the priority queue of the block does not inform the logic. If a
> datanode holds the last remaining replica of a block and has already reached
> its replication limit, the node is dismissed outright and the replication is
> not scheduled.
> In some situations, this could lead to data loss, as the last remaining
> replica could disappear if an opportunity is not taken to schedule a
> replication. It would be better to waive the max replication limit in cases
> of highest-priority block replication.
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