Re: pgsql: Compute XID horizon for page level index vacuum on primary.

On Fri, Mar 29, 2019 at 4:27 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On March 28, 2019 11:24:46 AM EDT, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> >On Thu, Mar 28, 2019 at 5:28 AM Andres Freund <andres(at)anarazel(dot)de>
> >wrote:
> >> Hm, good catch. I don't like this fix very much (even if it were
> >> commented), but I don't have a great idea right now.
> >
> >That was just a POC, to verify the problem. Not a proposal.
>
> I'm mildly inclined to push a commented version of this. And add a open items entry. Alternatively I'm thinking of just but taking the tablespace setting into account.

I didn't understand that last sentence.

Here's an attempt to write a suitable comment for the quick fix. And
I suppose effective_io_concurrency is a reasonable default.

It's pretty hard to think of a good way to get your hands on the real
value safely from here. I wondered if there was a way to narrow this
to just GLOBALTABLESPACE_OID since that's where pg_tablespace lives,
but that doesn't work, we access other catalog too in that path.

Hmm, it seems a bit odd that 0 is supposed to mean "disable issuance
of asynchronous I/O requests" according to config.sgml, but here 0
will prefetch 10 buffers.

--
Thomas Munro
https://enterprisedb.com