Re: VACUUM's ancillary tasks

On Sun, Oct 16, 2016 at 3:35 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> On Fri, Oct 7, 2016 at 6:14 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Thu, Oct 6, 2016 at 8:40 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> > In commit 37484ad2aacef5ec7, you changed the way that frozen tuples were
>> > represented, so that we could make freezing more aggressive without
>> > losing
>> > forensic evidence. But I don't think we ever did anything to actually
>> > make
>> > the freezing more aggressive.
>>
>> See 3cff1879f8d03cb729368722ca823a4bf74c0cac. The objection to doing
>> it in other cases is that it adds write-ahead log volume which might
>> cause its own share of problems. There might be some way of getting
>> ahead of that, though. I think if we piggyback on an existing WAL
>> record like XLOG_HEAP2_CLEAN or XLOG_HEAP2_VISIBLE the impact might be
>> minimal, but I haven't been dedicated enough to try to write the
>> patch.
>>
>> > When I applied the up-thread patch so that pgbench_history gets autovac,
>> > those autovacs don't actually cause any pages to get frozen until the
>> > wrap
>> > around kicks in, even when all the tuples on the early pages should be
>> > well
>> > beyond vacuum_freeze_min_age.
>>
>> If the pages are already all-visible, they'll be skipped until
>> vacuum_freeze_table_age is reached.
>
> So if I set vacuum_freeze_min_age to zero, then they should become
> all-visible and all-frozen at the same time, and avoid that problem?

Hmm. I *think* so...

> From the docs on vacuum_freeze_min_age: "Increasing this setting may avoid
> unnecessary work if the rows that would otherwise be frozen will soon be
> modified again". How much work is that? Presumably they are already getting
> marked visible, is marking them frozen too a meaningful amount of extra
> work? Is it just the extra WAL record?

Yeah, the extra WAL record is the main thing, I think.

--
Robert Haas
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