Re: Decoupling antiwraparound autovacuum from special rules around auto cancellation

On Tue, Jan 17, 2023 at 1:02 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2023-01-16 13:58:21 -0800, Peter Geoghegan wrote:
> > On Fri, Jan 13, 2023 at 9:55 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > When I express skepticism of very high autovacuum_freeze_max_age
> > settings, it's mostly just that I don't think that age(relfrozenxid)
> > is at all informative in the way that something that triggers
> > autovacuum ought to be. Worst of all, the older relfrozenxid gets, the
> > less informative it becomes. I'm sure that it can be safe to use very
> > high autovacuum_freeze_max_age values, but it seems like a case of
> > using a very complicated and unreliable thing to decide when to
> > VACUUM.
>
> Both you and Robert said this, and I have seen it be true, but typically not
> for large high-throughput OLTP databases, where I found increasing
> relfrozenxid to be important. Sure, there's probably some up/down through the
> day / week, but it's likely to be pretty predictable.
>
> I think the problem is that an old relfrozenxid doesn't tell you how much
> outstanding work there is. Perhaps that's what both of you meant...

I think so, at least in my case. The reason why the default
autovacuum_freeze_max_age is 300m is not because going over 300m is a
problem, but because we don't know how long it's going to take to run
all of the vacuums, and we're still going to be consuming XIDs in the
meantime, and we have no idea how fast we're consuming XIDs either.
For all we know it might take days, and we might be burning through
XIDs really quickly, so we might need a ton of headroom.

That's not to say that raising the setting might not be a sensible
thing to do in a context where you know what the workload is. If you
know that the vacuums will completely quickly OR that the XID burn
rate is low, you can raise it drastically and be fine. We just can't
assume that will be true everywhere.

> I think that's not the fault of relfrozenxid as a trigger, but that we simply
> don't keep enough other stats. We should imo at least keep track of:
>
> In pg_class:
>
> - The number of all frozen pages, like we do for all-visible
>
> That'd give us a decent upper bound for the amount of work we need to do to
> increase relfrozenxid. It's important that this is crash safe (thus no
> pg_stats), and it only needs to be written when we'd likely make other
> changes to the pg_class row anyway.

I'm not sure how useful this is because a lot of the work is from
scanning the indexes.

> In pgstats:
>
> - The number of dead items, incremented both by the heap scan and
> opportunistic pruning
>
> This would let us estimate how urgently we need to clean up indexes.

I don't think this is true because btree indexes are self-cleaning in
some scenarios and not in others.

> - The xid/mxid horizons during the last vacuum
>
> - The number of pages with tuples that couldn't removed due to the horizon
> during the last vacuum
>
> - The number of pages with tuples that couldn't be frozen

Not bad to know, but if the horizon we could use advances by 1, we
can't tell whether that allows pruning nothing additional or another
billion tuples.

I'm not trying to take the position that XID age is a totally useless
metric. I don't think it is. If XID age is high, you know you have a
problem, and the higher it is, the more urgent that problem is.
Furthemore, if XID is low, you know that you don't have that
particular problem. You might have some other one, but that's OK: this
one metric doesn't have to answer every question to be useful.
However, where XID age really falls down as a metric is that it
doesn't tell you what it's going to take to solve the problem. The
answer, at ten thousand feet, is always vacuum. But how long will that
vacuum run? We don't know. Do we need the XID horizon to advance
first, and if so, how far? We don't know. Do we need auto-cancellation
to be disabled? We don't know. That's where we get into a lot of
trouble here.

--
Robert Haas
EDB: http://www.enterprisedb.com