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Re: Checkpoint cost, looks like it is WAL/CRC

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: josh(at)agliodbs(dot)com
Cc: Postgres Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Checkpoint cost, looks like it is WAL/CRC
Date: 2005-06-30 03:33:02
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> 1. Offer a GUC to turn off full-page-image dumping, which you'd use only
>> if you really trust your hardware :-(

> Are these just WAL pages?  Or database pages as well?

Database pages.  The current theory is that we can completely
reconstruct from WAL data every page that's been modified since the
last checkpoint.  So the first write of any page after a checkpoint
dumps a full image of the page into WAL; subsequent writes only write

This is nice and secure ... at least when you are using hardware that
guarantees write ordering ... otherwise it's probably mostly useless
overhead.  Still, I'd not like to abandon the contract that if the disk
does what it is supposed to do then we will do what we are supposed to.

			regards, tom lane

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