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Re: PD_ALL_VISIBLE flag error on 9.0 alpha 4

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-bugs(at)postgresql(dot)org
Subject: Re: PD_ALL_VISIBLE flag error on 9.0 alpha 4
Date: 2010-03-10 23:26:06
Message-ID: 1268263566.3825.2554.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackers
On Wed, 2010-03-10 at 17:55 -0500, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> >>> Time to remove vacuum_defer_cleanup_age, I think.
> >> 
> >> Umm, so what's the bug?
> 
> > Whether you call it a bug or just an annoyance is debatable, but the
> > source of it is clear.
> 
> Maybe to you, but the rest of us would like to know.

If vacuum_defer_cleanup_age is set higher this causes the xmin to go
backwards, leading to the "PD_ALL_VISIBLE flag was incorrectly set"
warning. 

Having this false xmin move backwards doesn't endanger the standby,
since the xids arrive and are checked normally. If they stop arriving
that is fine.

Having the false xmin going backwards is not a serious issue on primary
because the actual xmin does not go backwards. No observer loses
information as a result of this, it is only about whether cleanup
records are generated later than normal, or not.

> > Given the lack of effectiveness, I propose
> > removing it.
> 
> I read Josh's recent report at
> http://archives.postgresql.org/message-id/4B973C3F.9070501@agliodbs.com
> to say that it's quite effective.  I think you're being way too hasty to
> decide that it can just be dropped.

OK, that's enough to not remove it. I was aware of more negative
thoughts and conscious of my own feelings about it being a kluge.

-- 
 Simon Riggs           www.2ndQuadrant.com


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