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https://issues.apache.org/jira/browse/HDFS-6840?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14093595#comment-14093595
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Andrew Wang commented on HDFS-6840:
-----------------------------------
Hey Jason, did you bisect this behavior to HDFS-6268? My impression of the old
pseudo-sort was that it was deterministic. AFAIK there wasn't a Random doing a
shuffle. The idea of not-shuffling by default was to preserve the old behavior,
and in case there were any nice page cache effects from directing reads to the
same replica.
> Clients are always sent to the same datanode when read is off rack
> ------------------------------------------------------------------
>
> Key: HDFS-6840
> URL: https://issues.apache.org/jira/browse/HDFS-6840
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 2.5.0
> Reporter: Jason Lowe
> Priority: Critical
>
> After HDFS-6268 the sorting order of block locations is deterministic for a
> given block and locality level (e.g.: local, rack. off-rack), so off-rack
> clients all see the same datanode for the same block. This leads to very
> poor behavior in distributed cache localization and other scenarios where
> many clients all want the same block data at approximately the same time.
> The one datanode is crushed by the load while the other replicas only handle
> local and rack-local requests.
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