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From Alain RODRIGUEZ <arodr...@gmail.com>
Subject Re: VNodes, Replication and Minimum cluster size
Date Thu, 10 Jan 2013 13:48:41 GMT
"I am curious to know if vnodes would change that or if that statement was
valid to begin with!"

This question was answered yesterday by Jonathan Ellis during the Datastax
C*ollege Webinar:
http://www.datastax.com/resources/webinars/whatsnewincassandra12 (about the
end of the video).

The answer is no. Vnodes doesn't change anything concerning the number of
nodes, the RF or the SPOF.

I don't know why you should start with 4 nodes, imho, 3 nodes should be
enough, or even 1 or 2 if you don't care about consistency or SPOF.

The point of having 3 nodes is that allow you to write & read using CL
quorum, which ensure you retrieving consistent data.
If a potential inconsistent data is not a problem for you, you can use 2
nodes RF = 2 and do both reads and writes with CL one, you'll have no SPOF.
You can work with one node but that's not really interesting since you
don't benefit from Cassandra at all.

With 4 nodes instead of 3 with a RF = 3, you start increasing your
performance because you don't have to write everything in every node. But
there is no need of starting with 4 nodes at all.

I hope I've been clear since english is not my mother tongue.

Alain


2013/1/10 Ryan Lowe <ryanjlowe@gmail.com>

> I have heard before that the recommended minimum cluster size is 4 (with
> replication factor of 3).  I am curious to know if vnodes would change that
> or if that statement was valid to begin with!
>
> The use case I am working on is one where we see tremendous amount of load
> for just 2 days out of the week and the rest of the time the cluster is
> pretty much idle.  It appears that vnodes will allow me to auto-scale the
> clusters size a little easier, but I am wondering what is the smallest I
> can get the cluster in physical server count and still have a good
> replication count.
>
> I'll panic about having 1 of 2 or 1 of 3 servers going down in an outage
> as a separate topic alone at night while not sleeping.
>
> Thanks!
> Ryan
>

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