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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, whole thread or download thread mbox)
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 

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