Subject: Re: Sync Rep Design
From: Simon Riggs <simon@2ndQuadrant.com>
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: heikki.linnakangas@enterprisedb.com, robertmhaas@gmail.com,
	greg@2ndquadrant.com, hannu@2ndquadrant.com, stefan@kaltenbrunner.cc,
	josh@postgresql.org, pgsql-hackers@postgresql.org
In-Reply-To: <4D2032840200002500038EC2@gw.wicourts.gov>
References: <4D2032840200002500038EC2@gw.wicourts.gov>
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Date: Sun, 02 Jan 2011 16:24:15 +0000
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On Sun, 2011-01-02 at 08:08 -0600, Kevin Grittner wrote:

> 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.

Thanks for those calculations.

Do you agree that requiring response from 2 sync standbys, or locking
up, gives us 94% server availability, but 99.9992% data durability? And
that adding additional async servers would not increase the server
availability of that cluster?

Now lets look at what happens when we first start a standby: we do the
base backup, configure the standby, it connects and then <wham> we
cannot process any new transactions until the standby has caught up,
which could well be hours on a big database. So if we don't have a
processing mode that allows work to continue, how will we ever enable
synchronous replication on a 24/7 database? How will we ever allow
standbys to catch up if they drop out for a while?

We should factor that into the availability calcs as well.

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