| 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 |
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| 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.
Regards,
Jeff Davis
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