Horton

Re: Logical to physical page mapping

Peter Geoghegan <peter@2ndquadrant.com>

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From: Peter Geoghegan <peter@2ndquadrant.com>
To: Jan Wieck <JanWieck@yahoo.com>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Development <pgsql-hackers@postgresql.org>
Date: 2012-10-29T02:50:27Z
Lists: pgsql-hackers 
On 28 October 2012 22:35, Jan Wieck <JanWieck@yahoo.com> wrote:
> The amount of WAL generated with full_page_writes=on is quite substantial.
> For pgbench for example the ratio 20:1. Meaning with full_page_writes you
> write 20x the amount you do without.

Sure, but as always, pgbench pessimises everything by having writes
follow a uniform distribution, which is completely unrepresentative of
the natural world. This will literally maximise the number of pristine
full page images that need to be included. The actual frequency of
checkpoints is a crucial factor here too, and you didn't mention
anything about that.


-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services