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From Justin Cameron <jus...@instaclustr.com>
Subject Re: Definition of QUORUM consistency level
Date Fri, 09 Jun 2017 04:51:37 GMT
Firstly, this situation only occurs if you need strong consistency and are
using an even replication factor (RF4, RF6, etc).
Secondly, either the read or write still need to be performed at a minimum
level of QUORUM. This means there are no extra availability benefits from
your proposal (i.e. a minimum of QUORUM replicas still need to be online
and available)

So the only potential benefit I can think of is a theoretical performance
boost. If you write with QUORUM, then you'll need to read with
QUORUM-1/HALF (e.g. RF4, write with QUORUM, read with TWO, RF6 write with
QUORUM, read with THREE, RF8 write with QUORUM, read with FOUR, ...). At
most you'd only reduce the number of replicas that the client needs to
block on by 1.

I'd guess that the performance benefits that you'd gain will probably be
quite small - but I'd happily be proven wrong if you feel like running some
benchmarks :)

Cheers,
Justin

On Fri, 9 Jun 2017 at 14:26 Brandon Williams <driftx@gmail.com> wrote:

> I don't disagree with you there and have never liked TWO/THREE.  This is
> somewhat relevant: https://issues.apache.org/jira/browse/CASSANDRA-2338
>
> I don't think going to CL.FOUR, etc, is a good long-term solution, but I'm
> also not sure what is.
>
>
> On Thu, Jun 8, 2017 at 11:20 PM, Dikang Gu <dikang85@gmail.com> wrote:
>
>> To me, CL.TWO and CL.THREE are more like work around of the problem, for
>> example, they do not work if the number of replicas go to 8, which does
>> possible in our environment (2 replicas in each of 4 DCs).
>>
>> What people want from quorum is strong consistency guarantee, as long as
>> R+W > N, there are three options: a) R=W=(n/2+1); b) R=(n/2), W=(n/2+1); c)
>> R=(n/2+1), W=(n/2). What Cassandra doing right now, is the option a), which
>> is the most expensive option.
>>
>> I can not think of a reason, that people want the quorum read, not for
>> strong consistency reason, but just to read from (n/2+1) nodes. If they
>> want strong consistency, then the read just needs (n/2) nodes, we are
>> purely waste the one extra request, and hurts read latency as well.
>>
>> Thanks
>> Dikang.
>>
>> On Thu, Jun 8, 2017 at 8:20 PM, Nate McCall <nate@thelastpickle.com>
>> wrote:
>>
>>>
>>> We have CL.TWO.
>>>>
>>>>
>>>>
>>> This was actually the original motivation for CL.TWO and CL.THREE if
>>> memory serves:
>>> https://issues.apache.org/jira/browse/CASSANDRA-2013
>>>
>>
>>
>>
>> --
>> Dikang
>>
>>
> --


*Justin Cameron*Senior Software Engineer


<https://www.instaclustr.com/>


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