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From "Alex Liu (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6268) Poor performance of Hadoop if any DC is using VNodes
Date Tue, 29 Oct 2013 21:45:26 GMT


Alex Liu commented on CASSANDRA-6268:

bq. Are there any valid use cases where we do like to run M/R job in more than one DC (e.g.
all)? describe_local_ring would disable that totally.

The local DC has at least one replica, so MR at local DC is sufficient for data locality.
describe_local_ring should return a virtual ring of the local DC using those replica.

> 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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