Simon Riggs wrote:
> On Sat, 2011-01-01 at 23:36 -0500, Robert Haas wrote:
>> On Sat, Jan 1, 2011 at 6:54 AM, Simon Riggs
wrote:
>>> Yes, working out the math is a good idea. Things are much clearer
>>> if we do that.
>>>
>>> Let's assume we have 98% availability on any single server.
>>>
>>> 1. Having one primary and 2 standbys, either of which can
>>> acknowledge, and we never lock up if both standbys fail, then we
>>> will have 99.9992% server availability. (So PostgreSQL hits "5
>>> Nines", with data guarantees). ("Maximised availability")
>>
>> I don't agree with this math. ...(snip by Simon)... 99.96%.
>
> OK, so that is at least 99.96%. Cool.
I think you're talking about different metrics, and you're both
right. With two servers configured in sync rep your chance of having
an available (running) server is 99.9992%. The chance that you know
that you have one that is totally up to date, with no lost
transactions is 99.9208%. The chance that you *actually* have
up-to-date data would be higher, but you'd have no way to be sure.
The 99.96% number is your certainty that you have a running server
with up-to-date data if only one machine is sync rep.
It's a matter of whether your shop needs five nines of availability
or the highest probability of not losing data. You get to choose.
-Kevin
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