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https://issues.apache.org/jira/browse/HDFS-4253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13532864#comment-13532864
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Colin Patrick McCabe commented on HDFS-4253:
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Thanks for clarifying that. I still think there's a problem, though-- I don't
see any reason why shuffle(a) could not be equal to shuffle(b), for two
completely unrelated DatanodeIDs a and b. This could be fixed by checking
something that's supposed to be unique in the case where the two agree-- like
the name field. It also seems better to just use {{hashCode}}, rather than
creating your own random set of random ints associated with objects.
> 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: hdfs4253-1.txt, hdfs4253-2.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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