| 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 |
| Message-ID: | 28288.1291136360@sss.pgh.pa.us |
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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
bits.
> 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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