Re: Berserk Autovacuum (let's save next Mandrill)

чт, 28 мар. 2019 г. в 01:01, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>:

> On 2019-Mar-28, Darafei "Komяpa" Praliaskouski wrote:
>
>
> > чт, 28 мар. 2019 г. в 00:32, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>:
> >
> > > On 2019-Mar-27, Darafei "Komяpa" Praliaskouski wrote:
>
> > > * certain tables would have some sort of partial scan that sets the
> > > visibility map. There's no reason to invoke the whole vacuuming
> > > machinery. I don't think this is limited to append-only tables, but
> > > rather those are just the ones that are affected the most.
> >
> > What other machinery runs on VACUUM invocation that is not wanted there?
> > Since Postgres 11 index cleanup is already skipped on append-only tables.
>
> Well, I think it would be useful to set all-visible earlier than waiting
> for a vacuum to be necessary, even for tables that are not append-only.
> So if you think about this just for the append-only table, you leave
> money on the table.
>

Thing is, problem does not exist for non-append-only tables, they're going
to be vacuumed after 50 rows got updated, automatically.

>
> > > * tables nearing wraparound danger should use the (yet to be committed)
> > > option to skip index cleaning, which makes the cleanup action faster.
> > > Again, no need for complete vacuuming.
> >
> > "Nearing wraparound" is too late already. In Amazon, reading table from
> gp2
> > after you exhausted your IOPS burst budget is like reading a floppy
> drive,
> > you have to freeze a lot earlier than you hit several terabytes of
> unfrozen
> > data, or you're dead like Mandrill's Search and Url tables from the link
> I
> > shared.
>
> OK, then start freezing tuples in the cheap mode (skip index updates)
> earlier than that. I suppose a good question is when to start.
>

Attached (autovacuum_berserk_v1.patch)
code achieves that. For append-only tables since
https://commitfest.postgresql.org/16/952/ vacuum skips index cleanup if no
updates happened. You just need to trigger it, and it already will be
"cheap".

--
Darafei Praliaskouski
Support me: http://patreon.com/komzpa