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Hadoop QA commented on HDFS-4253: --------------------------------- {color:green}+1 overall{color}. Here are the results of testing the latest attachment http://issues.apache.org/jira/secure/attachment/12555568/hdfs4253.txt against trunk revision . {color:green}+1 @author{color}. The patch does not contain any @author tags. {color:green}+1 tests included{color}. The patch appears to include 1 new or modified test files. {color:green}+1 javac{color}. The applied patch does not increase the total number of javac compiler warnings. {color:green}+1 javadoc{color}. The javadoc tool did not generate any warning messages. {color:green}+1 eclipse:eclipse{color}. The patch built with eclipse:eclipse. {color:green}+1 findbugs{color}. The patch does not introduce any new Findbugs (version 1.3.9) warnings. {color:green}+1 release audit{color}. The applied patch does not increase the total number of release audit warnings. {color:green}+1 core tests{color}. The patch passed unit tests in hadoop-common-project/hadoop-common hadoop-hdfs-project/hadoop-hdfs. {color:green}+1 contrib tests{color}. The patch passed contrib unit tests. Test results: https://builds.apache.org/job/PreCommit-HDFS-Build/3581//testReport/ Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/3581//console This message is automatically generated. > block replica reads get hot-spots due to NetworkTopology#pseudoSortByDistance > ----------------------------------------------------------------------------- > > Key: HDFS-4253 > URL: https://issues.apache.org/jira/browse/HDFS-4253 > Project: Hadoop HDFS > Issue Type: Bug > Affects Versions: 3.0.0, 2.0.2-alpha > Reporter: Andy Isaacson > Assignee: Andy Isaacson > Attachments: hdfs4253.txt > > > When many nodes (10) read from the same block simultaneously, we get > asymmetric distribution of read load. This can result in slow block reads > when one replica is serving most of the readers and the other replicas are > idle. The busy DN bottlenecks on its network link. > This is especially visible with large block sizes and high replica counts (I > reproduced the problem with {{-Ddfs.block.size=4294967296}} and replication > 5), but the same behavior happens on a small scale with normal-sized blocks > and replication=3. > The root of the problem is in {{NetworkTopology#pseudoSortByDistance}} which > explicitly does not try to spread traffic among replicas in a given rack -- > it only randomizes usage for off-rack replicas. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira