Date: Fri, 12 Jan 2024 16:48:20 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "Kumar, Sachin" <ssetiya@amazon.com>,
	Robins Tharakan <tharakan@gmail.com>, Jan Wieck <jan@wi3ck.info>,
	Bruce Momjian <bruce@momjian.us>, Zhihong Yu <zyu@yugabyte.com>,
	Andrew Dunstan <andrew@dunslane.net>,
	Magnus Hagander <magnus@hagander.net>,
	Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: pg_upgrade failing for 200+ million Large Objects
Message-ID: <20240112224820.GB3857154@nathanxps13>
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On Fri, Jan 05, 2024 at 03:02:34PM -0500, Tom Lane wrote:
> On further reflection, there is a very good reason why it's done like
> that.  Because pg_upgrade is doing schema-only dump and restore,
> there's next to no opportunity for parallelism within either pg_dump
> or pg_restore.  There's no data-loading steps, and there's no
> index-building either, so the time-consuming stuff that could be
> parallelized just isn't happening in pg_upgrade's usage.
> 
> Now it's true that my 0003 patch moves the needle a little bit:
> since it makes BLOB creation (as opposed to loading) parallelizable,
> there'd be some hope for parallel pg_restore doing something useful in
> a database with very many blobs.  But it makes no sense to remove the
> existing cross-database parallelism in pursuit of that; you'd make
> many more people unhappy than happy.

I assume the concern is that we'd end up multiplying the effective number
of workers if we parallelized both in-database and cross-database?  Would
it be sufficient to make those separately configurable with a note about
the multiplicative effects of setting both?  I think it'd be unfortunate if
pg_upgrade completely missed out on this improvement.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com