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https://issues.apache.org/jira/browse/HDFS-4253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14525440#comment-14525440
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Hadoop QA commented on HDFS-4253:
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| (x) *{color:red}-1 overall{color}* |
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|| Vote || Subsystem || Runtime || Comment ||
| {color:red}-1{color} | patch | 0m 0s | The patch command could not apply
the patch during dryrun. |
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|| Subsystem || Report/Notes ||
| Patch URL |
http://issues.apache.org/jira/secure/attachment/12729962/HDFS-4253.06.patch |
| Optional Tests | javadoc javac unit findbugs checkstyle |
| git revision | trunk / 6ae2a0d |
| Console output |
https://builds.apache.org/job/PreCommit-HDFS-Build/10721/console |
This message was 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: HDFS-4253.06.patch, hdfs4253-1.txt, hdfs4253-2.txt,
> hdfs4253-3.txt, hdfs4253-4.txt, hdfs4253-5.txt, hdfs4253-6.txt, 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.
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