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From "Jason Lowe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6840) Clients are always sent to the same datanode when read is off rack
Date Fri, 29 Aug 2014 21:31:54 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6840?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14115859#comment-14115859
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Jason Lowe commented on HDFS-6840:
----------------------------------

The test failures appear to be unrelated, and they pass for me locally with this patch applied.

Looks good overall, just one nit.  This comment was left in the code and no longer applies:

{code}
    // Seed is normally the block id
    // This means we use the same pseudo-random order for each block, for
    // potentially better page cache usage.
    // Seed is not used if we want to randomize block location for every block
{code}


> 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
>            Assignee: Andrew Wang
>            Priority: Critical
>         Attachments: hdfs-6840.001.patch, hdfs-6840.002.patch
>
>
> 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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