Subject: Re: New trigger option of pg_standby
From: Simon Riggs <simon@2ndQuadrant.com>
To: Fujii Masao <masao.fujii@gmail.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	Guillaume Smet <guillaume.smet@gmail.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, Peter Eisentraut <peter_e@gmx.net>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
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Date: Fri, 10 Apr 2009 17:31:43 +0100
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Fujii-san,

I like the new patch using the content of the file to determine the
mode. Much easier to use at failover time.

On Fri, 2009-04-10 at 12:47 +0900, Fujii Masao wrote:

> > One problem with this patch is that in smart mode, the trigger file is not
> > deleted. That's different from current pg_standby behavior, and makes
> > accidental failovers after one failover more likely.
> 
> Yes, it's because pg_standby cannot be sure when the trigger file
> can be removed in smart mode. If the trigger file is deleted as soon
> as it's found, just like in fast mode, pg_standby may keep waiting
> for WAL file again.

My understanding of smart mode is fairly simple: 

   if (triggered)
   {
	if (smartMode && nextWALfile+1 exists)
		exit(0);
	else
	{
		delete trigger file
		exit(1);
	}
   }

If you perform a file lookahead (the +1) as shown above then you avoid
the problem Heikki observes.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support