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Re: Sync Rep Design

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: heikki(dot)linnakangas(at)enterprisedb(dot)com, robertmhaas(at)gmail(dot)com, greg(at)2ndquadrant(dot)com, hannu(at)2ndquadrant(dot)com, stefan(at)kaltenbrunner(dot)cc, josh(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Sync Rep Design
Date: 2011-01-02 16:24:15
Message-ID: 1293985455.2090.1409.camel@ebony (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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 
 PostgreSQL Development, 24x7 Support, Training and Services

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Subject: Re: Base Backup Streaming
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Subject: Re: Sync Rep Design

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