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From Sam Overton <...@acunu.com>
Subject Re: VNodes, Replication and Minimum cluster size
Date Thu, 10 Jan 2013 13:53:44 GMT
On 10 January 2013 13:07, Ryan Lowe <ryanjlowe@gmail.com> wrote:
> 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 reason that RF=3 is recommended is that it is the minumum RF that
allows you to have both strong consistency and tolerate one node
failure (reading and writing at consistency level = QUORUM). With RF=2
for example you would have to choose between having strong consistency
(read/write at CL=QUORUM) or tolerating one node failure (read/write
at CL=ONE).

> 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,

The key advantage of vnodes in this case is that you do not need to
manually rebalance the cluster when adding or removing nodes.

> but I am wondering what is the smallest I can
> get the cluster in physical server count and still have a good replication
> count.

3 nodes at RF=3 would be the smallest advisable size. You could even
drop this to 2 nodes if you did not need consistency or availability
guarantees during the time the cluster is at the smallest size.

> 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.

Make sure that you over-provision sufficiently so that 2 nodes can
handle the load that 3 nodes would normally be taking in the event
that a node fails! (Or more generally, ensure N-1 nodes can handle the
load of N nodes). Testing a simulated load on a test cluster with one
node failure is always a good way to increase your confidence about
availability and explore any potential degradation in your client
application (latency etc.).

> Thanks!
> Ryan


-- 
Sam Overton
Acunu | http://www.acunu.com | @acunu

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