[ https://issues.apache.org/jira/browse/HADOOP-940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12469877 ] 

dhruba borthakur commented on HADOOP-940:
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if a block is in pendingReplication, then it means that this block needs more copies. It does not say anythung about how many more copies are to be made. When a heartbeat arirves at the namenode, the heartbeat processing code picks up a block from  neededReplication, determines how many new replicas to be made and then tells the datanode to start replication.

As part of hadoop-923, I am changing thet fact that heartbeat processing is doing all the heavy lifting. A new thread inside FSNamesystem will periodically pick up blocks from neededReplication and compute targets of new replicas.

But I agree that pendingReplication queue shoudl be periodically scanned and old items be dealt with appropriately.

> pendingReplications of FSNamesystem is not informative
> ------------------------------------------------------
>
>                 Key: HADOOP-940
>                 URL: https://issues.apache.org/jira/browse/HADOOP-940
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: dfs
>    Affects Versions: 0.10.1
>            Reporter: Hairong Kuang
>             Fix For: 0.11.0
>
>
> Currently when a neededReplication block is scheduled to be replicated, it is put to the pendingReplications queue. When it is no longer under replicated, it is pulled out of the pendingReplications queue. But the queue does not provide any information like how many targets have been choosen or who those targets are. PendingReplications are not used when deciding if a block is under replication. This may cause a block to be over replications or inaccurate estimate of its replication priority.
> For example, when a block has 1 replicas but it's replication factor is 2, a data node is choosen to replicate this block and the block is put in the pendingReplications queue. If the block's replication factor is changed to be 3 before the block replication notification, which is the next block report, comes in, the block will be put into neededReplictions queue again under the assumption that it needs to choose 2 targets instead of 1. So the block will end up with 4 replicas.
> I propose that we change pendingReplications to be a map from a block to the choosen data nodes. Data nodes in both pendingReplications and blockMap are used when deciding the total number of replicas that a block has. When the name node is notified that the block is replicated in a choosen data node, the data node is moved from pendingReplications to blockMap.
> Each choosen target is also associated with a timer indicating how long it expects to receive the block replication notification. PendingReplications queue needs to be periodically scanned to remove those data nodes whose timer is expired.

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