Re: crash-safe visibility map, take five

On Wed, 2011-06-22 at 21:53 -0400, Robert Haas wrote:
> On Wed, Jun 22, 2011 at 8:53 PM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> > On Thu, 2011-06-16 at 23:17 -0400, Noah Misch wrote:
> >> 2. In the words of a comment added by the patch:
> >> * The critical integrity requirement here is that we must never end up with
> >> * a situation where the visibility map bit is set, and the page-level
> >> * PD_ALL_VISIBLE bit is clear. If that were to occur, then a subsequent
> >> * page modification would fail to clear the visibility map bit.
> >> It does this by WAL-logging the operation of setting a vm bit. This also has
> >> the benefit of getting vm bits set correctly on standbys.
> >
> > In the same function, there is also the comment:
> >
> > "We don't bump the LSN of the heap page when setting the visibility
> > map bit, because that would generate an unworkable volume of
> > full-page writes. This exposes us to torn page hazards, but since
> > we're not inspecting the existing page contents in any way, we
> > don't care."
> >
> > It would be nice to have a comment explaining why that is safe with
> > respect to the WAL-before-data rule. Obviously torn pages aren't much of
> > a problem, because it's a single bit and completely idempotent.
>
> That's exactly what I was trying to explain in the comment you cite.
> Feel free to propose a specific change...

Well, I was a little unsure, but here's my attempt:

The potential problems are:
1. Torn pages -- not a problem because it's a single bit and idempotent.
2. PD_ALL_VISIBLE bit makes it to disk before a WAL record representing
an action that makes the page all-visible. Depending on what action
makes a page all-visible:
a. An old snapshot is released -- not a problem, because if there is a
crash all snapshots are released.
b. Cleanup action on the page -- not a problem, because that will
create a WAL record and update the page's LSN before setting the
PD_ALL_VISIBLE.

First of all, is that correct? Second, are there other cases to
consider?

Regards,
Jeff Davis