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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: Bruce Momjian <bruce(at)momjian(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-12-01 16:40:46
Message-ID: 17462.1291221646@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> As far as I can tell, there are basically two viable solutions on the
> table here.

> 1. Every time we observe a page as all-visible, (a) set the
> PD_ALL_VISIBLE bit on the page, without bumping the LSN; (b) set the
> bit in the visibility map page, bumping the LSN as usual, and (c) emit
> a WAL record indicating the relation and block number.  On redo of
> this record, set both the page-level bit and the visibility map bit.
> The heap page may hit the disk before the WAL record, but that's OK;

Um, no it isn't.  Suppose the heap page gets to disk but we crash before
the WAL record does.  Now we have a persistent state where the heap page
is marked PD_ALL_VISIBLE but the corresponding VM bit is not set.  The
VM bit will never become set, either, because operations on the heap
page will see PD_ALL_VISIBLE and assume it already is set.  This state
of affairs might be acceptable from a correctness standpoint, but not
from a performance standpoint.

			regards, tom lane

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Subject: Re: crash-safe visibility map, take three
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