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Re: crash-safe visibility map, take three

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-12-01 22:24:22
Message-ID: 1291242262.21972.35.camel@jdavis-ux.asterdata.local (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, 2010-12-01 at 15:59 -0500, Robert Haas wrote:
> As for CRCs, there's a pretty direct chain of inference here:
> 1. CRCs are hard (really impossible) because we have hint bits.

I would disagree with "impossible". If we don't set hint bits during
reading; and when we do set them, we log them (including full page
writes); then we can do CRCs.

Those things have costs, but we might be willing to pay them if we had a
bulk loading strategy that avoids or mitigates the costs.

The reason we can't do CRCs now is because hint bits violate the
WAL-before-data rule; not because of hint bits themselves. We're talking
about adding another feature that breaks the rule, in a more complex way
than hint bits.

I just wanted to step back for a second and consider the problem from a
different angle before we committed to that.

	Jeff Davis

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