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From "Branimir Lambov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7032) Improve vnode allocation
Date Fri, 26 Jun 2015 09:40:06 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7032?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14602624#comment-14602624
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Branimir Lambov commented on CASSANDRA-7032:
--------------------------------------------

Yes, if we change the vnode allocation to be done by this algorithm by default. Which may
be the right thing to do, but I don't know if we shouldn't be a bit cautious and let it be
tested in the real world first as a non-default.

> Improve vnode allocation
> ------------------------
>
>                 Key: CASSANDRA-7032
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7032
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Branimir Lambov
>              Labels: performance, vnodes
>             Fix For: 3.x
>
>         Attachments: TestVNodeAllocation.java, TestVNodeAllocation.java, TestVNodeAllocation.java,
TestVNodeAllocation.java, TestVNodeAllocation.java, TestVNodeAllocation.java
>
>
> It's been known for a little while that random vnode allocation causes hotspots of ownership.
It should be possible to improve dramatically on this with deterministic allocation. I have
quickly thrown together a simple greedy algorithm that allocates vnodes efficiently, and will
repair hotspots in a randomly allocated cluster gradually as more nodes are added, and also
ensures that token ranges are fairly evenly spread between nodes (somewhat tunably so). The
allocation still permits slight discrepancies in ownership, but it is bound by the inverse
of the size of the cluster (as opposed to random allocation, which strangely gets worse as
the cluster size increases). I'm sure there is a decent dynamic programming solution to this
that would be even better.
> If on joining the ring a new node were to CAS a shared table where a canonical allocation
of token ranges lives after running this (or a similar) algorithm, we could then get guaranteed
bounds on the ownership distribution in a cluster. This will also help for CASSANDRA-6696.



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