Re: problems with making relfilenodes 56-bits

Hi,

On 2022-10-03 19:40:30 +0200, Matthias van de Meent wrote:
> On Mon, 3 Oct 2022, 19:01 Andres Freund, <andres@anarazel.de> wrote:
> > Random idea: xl_prev is large. Store a full xl_prev in the page header, but
> > only store a 2 byte offset from the page header xl_prev within each record.
> 
> With that small xl_prev we may not detect partial page writes in
> recycled segments; or other issues in the underlying file system. With
> small record sizes, the chance of returning incorrect data would be
> significant for small records (it would be approximately the chance of
> getting a record boundary on the underlying page boundary * chance of
> getting the same MAXALIGN-adjusted size record before the persistence
> boundary). That issue is part of the reason why my proposed change
> upthread still contains the full xl_prev.

What exactly is the theory for this significant increase? I don't think
xl_prev provides a meaningful protection against torn pages in the first
place?

Greetings,

Andres Freund