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Re: testing HS/SR - 1 vs 2 performance

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Erik Rijkers <er(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-21 13:37:46
Message-ID: 1271857066.8305.27964.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, 2010-04-21 at 15:27 +0300, Heikki Linnakangas wrote:

> Given the discussion about the cyclic nature of XIDs, it would be good
> to add an assertion that when a new XID is added to the array, it is
> 
> a) larger than the biggest value already in the array
> (TransactionIdFollows(new, head)), and
> b) not too far from the smallest value in the array to confuse binary
> search (TransactionIdFollows(new, tail)).

We discussed this in November. You convinced me it isn't possible for
older xids to stay in the standby because anti-wraparound vacuums would
conflict and kick them out. The primary can't run with wrapped xids and
neither can the standby. I think that is correct.

Adding an assertion isn't going to do much because it's unlikely anybody
is going to be running for 2^31 transactions with asserts enabled.

Worrying about things like this seems strange when real and negative
behaviours are right in our faces elsewhere. Performance and scalability
are real world concerns.

-- 
 Simon Riggs           www.2ndQuadrant.com


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