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From Eric Evans <eev...@acunu.com>
Subject Re: RFC: Cassandra Virtual Nodes
Date Wed, 21 Mar 2012 13:50:00 GMT
On Tue, Mar 20, 2012 at 9:53 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
> It's reasonable that we can attach different levels of importance to
> these things.  Taking a step back, I have two main points:
>
> 1) vnodes add enormous complexity to *many* parts of Cassandra.  I'm
> skeptical of the cost:benefit ratio here.
>
> 1a) The benefit is lower in my mind because many of the problems
> solved by vnodes can be solved "well enough" for "most people," for
> some value of those two phrases, without vnodes.
>
> 2) I'm not okay with a "commit something half-baked and sort it out
> later" approach.

I must admit I find this a little disheartening.  The discussion has
barely started.  No one has had a chance to discuss implementation
specifics so that the rest of us could understand *how* disruptive it
would be (a necessary requirement in weighing cost:benefit), or what
an incremental approach would look like, and yet work has already
begun on shutting this down.

Unless I'm reading you wrong, your mandate (I say mandate because you
hinted at a veto elsewhere), is No to anything complex or invasive
(for some value of each).  The only alternative would seem to be a
phased or incremental approach, but you seem to be saying No to that
as well.

There seems to be quite a bit of interest in having virtual nodes (and
there has been for as long as I can remember), the only serious
reservations relate to the difficulty/complexity.  Is there really no
way to put our heads together and figure out how to properly manage
that aspect?

> On Tue, Mar 20, 2012 at 11:10 AM, Richard Low <rlow@acunu.com> wrote:
>> On 20 March 2012 14:55, Jonathan Ellis <jbellis@gmail.com> wrote:
>>> Here's how I see Sam's list:
>>>
>>> * Even load balancing when growing and shrinking the cluster
>>>
>>> Nice to have, but post-bootstrap load balancing works well in practice
>>> (and is improved by TRP).
>>
>> Post-bootstrap load balancing without vnodes necessarily streams more
>> data than is necessary.  Vnodes streams the minimal amount.
>>
>> In fact, post-bootstrap load balancing currently streams a constant
>> fraction of your data - the network traffic involved in a rebalance
>> increases linearly with the size of your cluster.  With vnodes it
>> decreases linearly.
>>
>> Including removing the ops overhead of running the load balance and
>> calculating new tokens, this makes removing post-bootstrap load
>> balancing a pretty big deal.
>>
>>> * Greater failure tolerance in streaming
>>>
>>> Directly addressed by TRP.
>>
>> Agreed.
>>
>>> * Evenly distributed impact of streaming operations
>>>
>>> Not a problem in practice with stream throttling.
>>
>> Throttling slows them down, increasing rebuild times so increasing downtime.
>>
>>> * Possibility for active load balancing
>>>
>>> Not really a feature of vnodes per se, but as with the other load
>>> balancing point, this is also improved by TRP.
>>
>> Again with the caveat that more data is streamed with TRP.  Vnodes
>> removes the need for any load balancing with RP.
>>
>>> * Distributed rebuild
>>>
>>> This is the 20% that TRP does not address.  Nice to have?  Yes.  Can I
>>> live without it?  I have so far.  Is this alone worth the complexity
>>> of vnodes?  No, it is not.  Especially since there are probably other
>>> approaches that we can take to mitigate this, one of which Rick has
>>> suggested in a separate sub-thread.
>>
>> Distributed rebuild means you can store more data per node with the
>> same failure probabilities.  This is frequently a limiting factor on
>> how much data you can store per node, increasing cluster sizes
>> unnecessarily.  I'd argue that this alone is worth the complexity of
>> vnodes.
>>
>> Richard.
>
>
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support
> http://www.datastax.com



-- 
Eric Evans
Acunu | http://www.acunu.com | @acunu

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