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Re: Sync Rep Design

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(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 16:54:29
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Lists: pgsql-hackers
On 02.01.2011 15:41, 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<simon(at)2ndquadrant(dot)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.

No-one is suggesting that to be the only option.

The "wait-for-all-to-ack" looks a lot less ridiculous if you also 
configure a timeout and don't wait for disconnected standbys. I'm not 
sure what the point of such a timeout in general is, but people have 
requested that. Also, setting synchronous_standbys="room1, room2" 
doesn't necessarily mean that you have just two standby servers, room1 
and room2 might both represent a group of servers.

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

   Heikki Linnakangas

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