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Re: Sync Rep Design

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, greg(at)2ndquadrant(dot)com, Josh Berkus <josh(at)postgresql(dot)org>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Sync Rep Design
Date: 2011-01-02 21:19:46
Message-ID: 1294003186.2090.3338.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, 2011-01-02 at 18:54 +0200, Heikki Linnakangas wrote:

> I believe we all agree that there's different use cases that require 
> different setups. Both "first-past-the-post" and "wait-for-all-to-ack" 
> have their uses. 

Robert's analysis is that "first-past-the-post" doesn't actually improve
the durability guarantee (according to his calcs). Which means that
  1 primary, 2 sync standbys with first-past-the-post
is actually worse than
  1 primary, 1 sync and 1 async standby
in terms of its durability guarantees.

So ISTM that Robert does not agree that both have their uses.

> I'm not 
> sure what the point of such a timeout in general is, but people have 
> requested that. 

Again, this sounds like you think a timeout has no measurable benefit,
other than to please some people's perceived needs.

> The "wait-for-all-to-ack" looks a lot less ridiculous if you also 
> configure a timeout and don't wait for disconnected standbys

Does it? Do Robert, Stefan and Aidan agree? What are the availability
and durability percentages if we do that? Based on those, we may decide
to do that instead. But I'd like to see some analysis of your ideas, not
just a "we could". Since nobody has commented on my analysis, lets see
someone else's.

> There's no point in arguing over which is better.

I'm trying to compare quantifiable benefit of various options to see
what goes into Postgres. I don't want to put anything in that we cannot
all agree has a measurable benefit to someone (that has the appropriate
preference).

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


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