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From Piotr Kołaczkowski (JIRA) <>
Subject [jira] [Commented] (CASSANDRA-6268) Poor performance of Hadoop if any DC is using VNodes
Date Tue, 29 Oct 2013 20:07:26 GMT


Piotr Kołaczkowski commented on CASSANDRA-6268:

I know, but LOCAL_ONE does not fix the problem. Actually the DC restriction is here so we
don't go to the DCs with enabled vnodes, as their ranges are split. If I could get the name
of the local DC from the place where splits are generated, then I could make it without this

> Poor performance of Hadoop if any DC is using VNodes
> ----------------------------------------------------
>                 Key: CASSANDRA-6268
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Hadoop
>            Reporter: Piotr Kołaczkowski
>            Assignee: Piotr Kołaczkowski
>         Attachments: 0001-DSP-2572-Adds-ability-to-set-target-DCs-where-a-Hado.patch
> Some customers are complaining about huge number of splits in Hadoop caused by VNodes.
Disabling vnodes only in Hadoop DC does not fix it. Splits are generated from the results
of describe_ring, which returns a huge number of ranges anyways, and doesn't take into account
that there will be huge number of consecutive ranges residing on the nodes we'd like the M/R
job to be run.
> The proposed fix:
> 1. allows for specifying the DC(s) the Hadoop job should be run in (in DSE - defaults
to all Hadoop DCs)
> 2. merges consecutive ranges before generating Hadoop splits, so we don't have artificial
range splitting caused by vnodes in the other DCs
> For non-DSE users this feature is turned off by default and doesn't change the old behaviour.

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