Re: crash-safe visibility map, take five

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.

Regards,
Jeff Davis