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https://issues.apache.org/jira/browse/HBASE-16212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15383565#comment-15383565
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Zhihua Deng commented on HBASE-16212:
-------------------------------------
Changing from threadlocal to a common field, yes there will be a potential
synchronization bottleneck, but it better than io operation. So the question
here is that how often the connection will be recreated for seeking + reading?
The original threadlocal is declared as non static private field here, it
means that the created fsreaderimpl instance will be reused later on, also a
inputstream is initiated when fsreaderimpl created.
Taken the case described in the attached log, The synchronization way is more
better than threadlocal.
How about concurrent case? the worst case: Thread1.readBlockInternal ->
Thread2.readBlockInternal -> Thread3.readBlockInternal ->
Thread1.readBlockInternal -> ....
In this case, the synchronization way is equal to threadlocal when taken how
many connections will be created into consideration.
> Many connections to datanode are created when doing a large scan
> -----------------------------------------------------------------
>
> Key: HBASE-16212
> URL: https://issues.apache.org/jira/browse/HBASE-16212
> Project: HBase
> Issue Type: Improvement
> Affects Versions: 1.1.2
> Reporter: Zhihua Deng
> Attachments: HBASE-16212.patch, HBASE-16212.v2.patch,
> regionserver-dfsinputstream.log
>
>
> As described in https://issues.apache.org/jira/browse/HDFS-8659, the datanode
> is suffering from logging the same repeatedly. Adding log to DFSInputStream,
> it outputs as follows:
> 2016-07-10 21:31:42,147 INFO
> [B.defaultRpcServer.handler=22,queue=1,port=16020] hdfs.DFSClient:
> DFSClient_NONMAPREDUCE_1984924661_1 seek
> DatanodeInfoWithStorage[10.130.1.29:50010,DS-086bc494-d862-470c-86e8-9cb7929985c6,DISK]
> for BP-360285305-10.130.1.11-1444619256876:blk_1109360829_35627143. pos:
> 111506876, targetPos: 111506843
> ...
> As the pos of this input stream is larger than targetPos(the pos trying to
> seek), A new connection to the datanode will be created, the older one will
> be closed as a consequence. When the wrong seeking ops are large, the
> datanode's block scanner info message is spamming logs, as well as many
> connections to the same datanode will be created.
> hadoop version: 2.7.1
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