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From "jack levin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6022) Moving deadNodes from being thread local. Improving dead datanode handling in DFSClient
Date Tue, 04 Mar 2014 06:20:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6022?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13919050#comment-13919050
] 

jack levin commented on HDFS-6022:
----------------------------------

AFAIK, (correct me if I am wrong), 

 static DatanodeInfo bestNode(...) is the only place where deadNodes list is actually used.

DeadNodes being global does not change retry logic, or changes how checksum errors are handled.
 The only real difference is moving the list management outside of DFSIntputstream.  

Can I ask what is the benefit of deadNodes being thread local?  It is possible that getting
a checksum will add the DN to deadNodes and remove it as candidate for other threads, however
that in itself is not a bad thing, since deadnodesverifier thread will purge the list within
less than 5 seconds.  The patch is really minimal, but it does improve things greatly in our
100 node hbase cluster.



> Moving deadNodes from being thread local. Improving dead datanode handling in DFSClient

> ----------------------------------------------------------------------------------------
>
>                 Key: HDFS-6022
>                 URL: https://issues.apache.org/jira/browse/HDFS-6022
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0, 0.23.9, 0.23.10, 2.2.0, 2.3.0
>            Reporter: Jack Levin
>            Assignee: Colin Patrick McCabe
>              Labels: patch
>             Fix For: 3.0.0, 2.3.0
>
>         Attachments: HADOOP-6022.patch
>
>   Original Estimate: 0h
>  Remaining Estimate: 0h
>
> This patch solves an issue of deadNodes list being thread local.  deadNodes list is created
by DFSClient when some problems with write/reading, or contacting datanode exist.  The problem
is that deadNodes is not visible to other DFSInputStream threads, hence every DFSInputStream
ends up building its own deadNodes.  This affect performance of DFSClient to a large degree
especially when a datanode goes completely offline (there is a tcp connect delay experienced
by all DFSInputStream threads affecting performance of the whole cluster).
> This patch moves deadNodes to be global in DFSClient class so that as soon as a single
DFSInputStream thread reports a dead datanode, all other DFSInputStream threads are informed,
negating the need to create their own independent lists (concurrent Map really). 
> Further, a global deadNodes health check manager thread (DeadNodeVerifier) is created
to verify all dead datanodes every 5 seconds, and remove the same list as soon as it is up.
 That thread under normal conditions (deadNodes empty) would be sleeping.  If deadNodes is
not empty, the thread will attempt to open tcp connection every 5 seconds to affected datanodes.
> This patch has a test (TestDFSClientDeadNodes) that is quite simple, since the deadNodes
creation is not affected by the patch, we only test datanode removal from deadNodes by the
health check manager thread.  Test will create a file in dfs minicluster, read from the same
file rapidly, cause datanode to restart, and test is the health check manager thread does
the right thing, removing the alive datanode from the global deadNodes list.



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