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Re: crash-safe visibility map, take three

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-11-30 16:59:20
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Nov 30, 2010 at 11:40 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Ouch. That seems like it could shoot down all these proposals. There
>> definitely isn't any way to make VM crash-safe if there is no WAL-driven
>> mechanism for setting the bits.

> Heikki's intent method works fine, because the WAL record only clears
> the visibility map bits on redo; it never sets them.

Uh, no, because he also had that final WAL record that would set the

> We could actually allow the slave to set the visibility map bits based
> on its own xmin horizon.

Not in a crash-safe way, which is exactly the problem here.

			regards, tom lane

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