On Tue, 2020-03-17 at 18:02 -0700, Andres Freund wrote:
> I don't think a default scale factor of 0 is going to be ok. For
> large-ish tables this will basically cause permanent vacuums. And it'll
> sometimes trigger for tables that actually coped well so far. 10 million
> rows could be a few seconds, not more.
>
> I don't think that the argument that otherwise a table might not get
> vacuumed before autovacuum_freeze_max_age is convincing enough.
>
> a) if that's indeed the argument, we should increase the default
> autovacuum_freeze_max_age - now that there's insert triggered vacuums,
> the main argument against that from before isn't valid anymore.
>
> b) there's not really a good arguments for vacuuming more often than
> autovacuum_freeze_max_age for such tables. It'll not be not frequent
> enough to allow IOS for new data, and you're not preventing
> anti-wraparound vacuums from happening.
According to my reckoning, that is the remaining objection to the patch
as it is (with ordinary freezing behavior).
How about a scale_factor od 0.005? That will be high enough for large
tables, which seem to be the main concern here.
I fully agree with your point a) - should that be part of the patch?
I am not sure about b). In my mind, the objective is not to prevent
anti-wraparound vacuums, but to see that they have less work to do,
because previous autovacuum runs already have frozen anything older than
vacuum_freeze_min_age. So, assuming linear growth, the number of tuples
to freeze during any run would be at most one fourth of today's number
when we hit autovacuum_freeze_max_age.
I am still sorry to see more proactive freezing go, which would
reduce the impact for truly insert-only tables.
After sleeping on it, here is one last idea.
Granted, freezing with vacuum_freeze_min_age = 0 poses a problem
for those parts of the table that will receive updates or deletes.
But what if insert-triggered vacuum operates with - say -
one tenth of vacuum_freeze_min_age (unless explicitly overridden
for the table)? That might still be high enough not to needlessly
freeze too many tuples that will still be modified, but it will
reduce the impact on insert-only tables.
Yours,
Laurenz Albe
