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https://issues.apache.org/jira/browse/HADOOP-3673?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12610077#action_12610077
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Sameer Paranjpye commented on HADOOP-3673:
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Note that while the lowest Datanode should be orchestrating block recovery the
Datanode order for *data* should *not* change. Data should go continue to the
closest node first, if not it could make unnecessary cross rack trips.
> Deadlock in Datanode RPC servers
> --------------------------------
>
> Key: HADOOP-3673
> URL: https://issues.apache.org/jira/browse/HADOOP-3673
> Project: Hadoop Core
> Issue Type: Bug
> Components: dfs
> Affects Versions: 0.18.0
> Reporter: dhruba borthakur
> Assignee: Tsz Wo (Nicholas), SZE
> Priority: Blocker
> Fix For: 0.18.0
>
> Attachments: 3673_20080702.patch
>
>
> There is a deadlock scenario in the way Lease Recovery is triggered using the
> Datanode RPC server via HADOOP-3310.
> Each Datanode has dfs.datanode.handler.count handler threads (default of 3).
> These handler threads are used to support the generation-stamp-dance protocol
> as described in HADOOP-1700.
> Let me try to explain the scenario with an example. Suppose, a cluster has
> two datanodes. Also, let's assume that dfs.datanode.handler.count is set to
> 1. Suppose that there are two clients, each writing to a separate file with a
> replication factor of 2. Let's assume that both clients encounter an IO error
> and triggers the generation-stamp-dance protocol. The first client may invoke
> recoverBlock on the first datanode while the second client may invoke
> recoverBlock on the second datanode. Now, each of the datanode will try to
> make a getBlockMetaDataInfo() to the other datanode. But since each datanode
> has only 1 server handler threads, both threads will block for eternity.
> Deadlock!
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