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From: Simon Riggs <simon@2ndQuadrant.com>
To: Aidan Van Dyk <aidan@highrise.ca>
Cc: Markus Wanner <markus@bluegap.ch>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: MARK CALLAGHAN <mdcallag@gmail.com>
Cc: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Andrew Dunstan <andrew@dunslane.net>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: Fri, 18 Mar 2011 21:26:00 +0000
Message-ID: <1300483560.18619.19276.camel@ebony> (raw)
In-Reply-To: <AANLkTimdn_yVBwsNXn9bnOvDmhxHKyLNBqJYab_Oji4z@mail.gmail.com>
References: <E1PwMns-00047O-UU@gemulon.postgresql.org>
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	<4D832571.5030808@bluegap.ch>
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	<4D832B4D020000250003BA98@gw.wicourts.gov>
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	<4D83B02A.5090105@bluegap.ch>
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	<AANLkTimdn_yVBwsNXn9bnOvDmhxHKyLNBqJYab_Oji4z@mail.gmail.com>

On Fri, 2011-03-18 at 17:08 -0400, Aidan Van Dyk wrote:
> On Fri, Mar 18, 2011 at 3:41 PM, Markus Wanner <markus@bluegap.ch> wrote:
> > On 03/18/2011 08:29 PM, Simon Riggs wrote:
> >> We could do that easily enough, actually, if we wished.
> >>
> >> Do we wish?
> >
> > I personally don't see any problem letting a standby show a snapshot
> > before the master.  I'd consider it unneeded network traffic.  But then
> > again, I'm completely biased.
> 
> In fact, we *need* to have standbys show a snapshot before the master.
> 
> By the time the master acks the commit to the client, the snapshot
> must be visible to all client connected to both the master and the
> syncronous slave.
> 
> Even with just a single server postgresql cluster, other
> clients(backends) can see the commit before the commiting client
> receives the ACK.  Just that on a single server, the time period for
> that is small.
> 
> Sync rep increases that time period by the length of time from when
> the slave reaches the commit point in the WAL stream to when it's ack
> of that point get's back to the wal sender.  Ideally, that ACK time is
> small.
> 
> Adding another round trip in there just for a "go almost to $COMIT,
> ok, now go to $COMMIT" type of WAL/ack is going to be pessimal for
> performance, and still not improve the *guarentees* it can make.
> 
> It can only slightly reduce, but not eliminated that window where them
> master has WAL that the slave doesn't, and without a complete
> elimination (where you just switch the problem to be the slave has the
> data that the master doesn't), you haven't changed any of the
> guarantees sync rep can make (or not).

Well explained observation. Agreed.

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




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  Cc: simon@2ndQuadrant.com, aidan@highrise.ca, markus@bluegap.ch, heikki.linnakangas@enterprisedb.com, Kevin.Grittner@wicourts.gov, mdcallag@gmail.com, alvherre@commandprompt.com, andrew@dunslane.net, pgsql-hackers@postgresql.org
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