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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Virtual Nodes, lots of physical nodes and potentially increasing outage count?
Date Mon, 10 Dec 2012 15:00:28 GMT
Assuming you need to work with quorum in a non-vnode scenario. That means
that if 2 nodes in a row in the ring are down some number of quorum
operations will fail with UnavailableException (TimeoutException right
after the failures). This is because the for a given range of tokens quorum
will be impossible, but quorum will be possible for others.

In a vnode world if any two nodes are down,  then the intersection of vnode
token ranges they have are unavailable.

I think it is two sides of the same coin.


On Mon, Dec 10, 2012 at 7:41 AM, Richard Low <rlow@acunu.com> wrote:

> Hi Tyler,
>
> You're right, the math does assume independence which is unlikely to be
> accurate.  But if you do have correlated failure modes e.g. same power,
> racks, DC, etc. then you can still use Cassandra's rack-aware or DC-aware
> features to ensure replicas are spread around so your cluster can survive
> the correlated failure mode.  So I would expect vnodes to improve uptime in
> all scenarios, but haven't done the math to prove it.
>
> Richard.
>

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