Re: [PROPOSAL] VACUUM Progress Checker.

On Fri, Dec 11, 2015 at 1:25 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
>> For another thing, there are definitely going to be
>> some people that want the detailed information - and I can practically
>> guarantee that if we don't make it available, at least one person will
>> write a tool that tries to reverse-engineer the detailed progress
>> information from whatever we do report.
>
> OK, so this justifies the fact of having detailed information, but
> does it justify the fact of having precise and accurate data? ISTM
> that having detailed information and precise information are two
> different things. The level of details is defined depending on how
> verbose we want the information to be, and the list you are giving
> would fulfill this requirement nicely for VACUUM. The level of
> precision/accuracy at which this information is provided though
> depends at which frequency we want to send this information. For
> long-running VACUUM it does not seem necessary to update the fields of
> the progress tracker each time a counter needs to be incremented.
> CLUSTER has been mentioned as well as a potential target for the
> progress facility, but it seems that it enters as well in the category
> of things that would need to be reported on a slower frequency pace
> than "each-time-a-counter-is-incremented".
>
> My impression is just based on the needs of VACUUM and CLUSTER.
> Perhaps I am lacking imagination regarding the potential use cases of
> the progress facility though in cases where we'd want to provide
> extremely precise progress information :)
> It just seems to me that this is not a requirement for VACUUM or
> CLUSTER. That's all.

It's not a hard requirement, but it should be quite easy to do without
adding any significant overhead. All you need to do is something
like:

foo->changecount++;
pg_write_barrier();
foo->count_of_blocks++;
pg_write_barrier();
foo->changecount++;

I suspect that's plenty cheap enough to do for every block.

--
Robert Haas
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