Re: crash-safe visibility map, take five

On Fri, May 6, 2011 at 5:47 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Mar 30, 2011 at 8:52 AM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> Another question:
>>> To address the problem in
>>> http://archives.postgresql.org/pgsql-hackers/2010-02/msg02097.php
>>> , should we just clear the vm before the log of insert/update/delete?
>>> This may reduce the performance, is there another solution?
>>
>> Yeah, that's a straightforward way to fix it. I don't think the performance
>> hit will be too bad. But we need to be careful not to hold locks while doing
>> I/O, which might require some rearrangement of the code. We might want to do
>> a similar dance that we do in vacuum, and call visibilitymap_pin first, then
>> lock and update the heap page, and then set the VM bit while holding the
>> lock on the heap page.
>
> Here's an attempt at implementing the necessary gymnastics.

Is there a quick synopsis of why you have to do (sometimes) the
pin->lock->unlock->pin->lock mechanic? How come you only can fail to
get the pin at most once?

merlin