To: Elliot Chance <elliotchance@gmail.com>
cc: pgsql-admin@postgresql.org
Subject: Re: pg_dump and XID limit 
In-reply-to: <DC1E3319-08CD-4C61-9565-06D744260261@gmail.com> 
References: <DC1E3319-08CD-4C61-9565-06D744260261@gmail.com>
Comments: In-reply-to Elliot Chance <elliotchance@gmail.com>
	message dated "Wed, 24 Nov 2010 13:18:28 +1100"
Date: Wed, 24 Nov 2010 01:07:49 -0500
Message-ID: <6420.1290578869@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Elliot Chance <elliotchance@gmail.com> writes:
> This is a hypothetical problem but not an impossible situation. Just curious about what would happen.

> Lets say you have an OLTP server that keeps very busy on a large database. In this large database you have one or more tables on super fast storage like a fusion IO card which is handling (for the sake of argument) 1 million transactions per second.

> Even though only one or a few tables are using almost all of the IO, pg_dump has to export a consistent snapshot of all the tables to somewhere else every 24 hours. But because it's such a large dataset (or perhaps just network congestion) the daily backup takes 2 hours.

> Heres the question, during that 2 hours more than 4 billion transactions could of occurred - so what's going to happen to your backup and/or database?

The DB will shut down to prevent wraparound once it gets 2 billion XIDs
in front of the oldest open snaphot.

			regards, tom lane