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

Hi,

On 2020-03-16 20:49:43 +0100, Laurenz Albe wrote:
> On Mon, 2020-03-16 at 07:47 -0500, Justin Pryzby wrote:
> > It seems to me that the easy thing to do is to implement this initially without
> > FREEZE (which is controlled by vacuum_freeze_table_age), and defer until
> > July/v14 further discussion and implementation of another GUC/relopt for
> > autovacuum freezing to be controlled by insert thresholds (or ratio).
>
> Freezing tuples is the point of this patch.

Sure. But not hurting existing installation is also a goal of the
patch. Since this is introducing potentially significant performance
downsides, I think it's good to be a bit conservative with the default
configuration.

I'm gettin a bit more bullish on implementing some of what what I
discussed in
https://www.postgresql.org/message-id/20200313213851.ejrk5gptnmp65uoo%40alap3.anarazel.de
at the same time as this patch.

In particularl, I think it'd make sense to *not* have a lower freezing
horizon for insert vacuums (because it *will* cause problems), but if
the page is dirty anyway, then do the freezing even if freeze_min_age
etc would otherwise prevent us from doing so?

It'd probably be ok to incur the WAL logging overhead unconditionally,
but I'm not sure about it.

> As I have said, if you have a table where you insert many rows in few
> transactions, you would trigger an autovacuum that then ends up doing nothing
> because none of the rows have reached vacuum_freeze_table_age yet.

> Then some time later you will get a really large vacuum run.

Well, only if you don't further insert into the table. Which isn't that
common a case for a table having a "really large vacuum run".

Greetings,

Andres Freund