[jira] Commented: (HADOOP-4643) NameNode should exclude excessive replicas when counting live replicas for a block

["Hairong Kuang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-4643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12647031#action_12647031 ] 

Hairong Kuang commented on HADOOP-4643:
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A patch to HADOOP-4540 will fix the problem described in this jira. But while I am working on HADOOP-4540, I realize that the policy change proposed in HADOOP-4540 bring subtle dfs semantics change which needs more discussion. So I plan to mark HADOOP-4540 to be fixed in 0.20 and having a simple solution in this jira to be fixed in 0.18. This jira is a part of the solution to avoid data loss as described in HADOOP-4556.

> NameNode should exclude excessive replicas when counting live replicas for a block
> ----------------------------------------------------------------------------------
>
>                 Key: HADOOP-4643
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4643
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.18.3
>
>
> Currently NameNode include excessive replicas in blockMap and count them as live replicas. Although excessive replicas have marked as invalid, scheduling deletion may be delayed and also datanode does not send deletion confirmation until the next block report. As a result, excessive replicas may stay in blocksMap for quite a while. This may cause underReplicated blocks undetected in NameNode. 
> For example, assume that block b is at datanode d1, d2, and d3. We have the following scenario
> 1. d1 loses heartbeat, NN will replicate b to another datanode, assuming it is d4. 
> 2. d1 comes back. NN finds out b is over-replicated therefore choosing one replica, assuming d4, as a excessive replica and marking it as invalid. Now b has 3 valid replicas d1, d2, d3 and 1 excessive (invalid) replica d4, all in blocksMap.
> 3. d2 loses heartbeat. d2 gets removed from blocksMap. Block b has 2 valid replicas d1 and d3, and 1 excessive invalid replica d4. Block b is under-replicated; But NN still counts block b has 3 live replicas so does not take any action to replicate block b.

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