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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-3673) Deadlock in Datanode RPC servers
Date Tue, 01 Jul 2008 00:30:44 GMT
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
            Reporter: dhruba borthakur

There is a deadlock scenario in the way Lease Recovery is triggered using the Datanode RPC
server via HADOOP-3283.

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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