Re: crash-safe visibility map, take five

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...

--
Robert Haas
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