Re: WAL_LOG CREATE DATABASE strategy broken for non-standard page layouts

On Mon, 13 May 2024 at 16:13, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com> writes:
> > PFA a patch that fixes this issue, by assuming that all pages in the
> > source database utilize a non-standard page layout.
>
> Surely that cure is worse than the disease?

I don't know where we would get the information whether the selected
relation fork's pages are standard-compliant. We could base it off of
the fork number (that info is available locally) but that doesn't
guarantee much.
For VM and FSM-pages we know they're essentially never
standard-compliant (hence this thread), but for the main fork it is
anyone's guess once the user has installed an additional AM - which we
don't detect nor pass through to the offending
RelationCopyStorageUsingBuffer.

As for "worse", the default template database is still much smaller
than the working set of most databases. This will indeed regress the
workload a bit, but only by the fraction of holes in the page + all
FSM/VM data.
I think the additional WAL volume during CREATE DATABASE is worth it
when the alternative is losing that data with physical
replication/secondary instances. Note that this does not disable page
compression, it just stops the logging of holes in pages; holes which
generally are only a fraction of the whole database.

It's not inconceivable that this will significantly increase WAL
volume, but I think we should go for correctness rather than fastest
copy. If we went with fastest copy, we'd better just skip logging the
FSM and VM forks because we're already ignoring the data of the pages,
so why not ignore the pages themselves, too? I don't think that holds
water when we want to be crash-proof in CREATE DATABASE, with a full
data copy of the template database.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)