Re: New IndexAM API controlling index vacuum strategies

Hi,

On 2021-04-14 21:30:29 -0700, Peter Geoghegan wrote:
> I think that this was once true, but is now much less common, mostly
> due to the freeze map stuff in 9.6. And due a general recognition that
> the *risk* of increasing them is just too great (a risk that we can
> hope was diminished by the failsafe, incidentally). As an example of
> this, Christophe Pettus had a Damascene conversion when it came to
> increasing autovacuum_freeze_max_age aggressively, which we explains
> here:
>
> https://thebuild.com/blog/2019/02/08/do-not-change-autovacuum-age-settings/

Not at all convinced. The issue of needing to modify a lot of
all-visible pages again to freeze them is big enough to let it be a
problem even after the freeze map. Yes, there's workloads where it's
much less of a problem, but not all the time.

> As I said, we handle the case where autovacuum_freeze_max_age is set
> to something larger than vacuum_failsafe_age is a straightforward and
> pretty sensible way. I am curious, though: what
> autovacuum_freeze_max_age setting is "much higher" than 1.6 billion,
> but somehow also not extremely ill-advised and dangerous? What number
> is that, precisely? Apparently this is common, but I must confess that
> it's the first I've heard about it.

I didn't intend to say that the autovacuum_freeze_max_age would be set
much higher than 1.6 billion, just that that the headroom would be much
less. I've set it, and seen it set, to 1.5-1.8bio without problems,
while reducing overhead substantially.

Greetings,

Andres Freund