Subject: Re: Sync Rep Design
From: Simon Riggs <simon@2ndQuadrant.com>
To: Robert Haas <robertmhaas@gmail.com>,
	Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: Hannu Krosing <hannu@2ndquadrant.com>, greg@2ndquadrant.com,
	Josh Berkus <josh@postgresql.org>,
	Stefan Kaltenbrunner <stefan@kaltenbrunner.cc>,
	pgsql-hackers@postgresql.org
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Date: Sun, 02 Jan 2011 13:41:13 +0000
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On Sat, 2011-01-01 at 23:36 -0500, Robert Haas wrote:
> On Sat, Jan 1, 2011 at 6:54 AM, Simon Riggs <simon@2ndquadrant.com> 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.

The key point here is not (1), but option (4).

The approach advocated by Heikki and yourself gives us 94% availability.
IMHO that is ridiculous, and I will not accept that as the *only* way
forwards, for that reason, whoever advocates it or for how long they
keep arguing. I do accept that some wish that as an option.

If we are to have a sensible technical debate with an eventual end, you
must answer the points placed in front of you, not just sidestep and try
to point out problems somewhere else. All analysis must be applied to
all options, not just those options advocated by someone else. I've been
asking for a failure mode analysis for months and it never comes in
full.

I'm more than happy to discuss your additional points once we are clear
on the 94% because it is pivotal to everything I've been proposing.

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services