Re: [PING] [PATCH v2] parallel pg_restore: avoid disk seeks when jumping short distance forward - Mailing list pgsql-hackers

On Wed, Jun 11, 2025 at 9:48 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
> So, fseeko() starts winning around 4096 bytes.  On macOS, the differences
> aren't quite as dramatic, but 4096 bytes is the break-even point there,
> too.  I imagine there's a buffer around that size somewhere...

BTW you can call setvbuf(f, my_buffer, _IOFBF, my_buffer_size) to
control FILE buffering.  I suspect that glibc ignores the size if you
pass NULL for my_buffer, so you'd need to allocate it yourself and it
should probably be aligned on PG_IO_ALIGN_SIZE for best results
(minimising the number of VM pages that must be held/pinned).  Then
you might be able to get better and less OS-dependent results.  I
haven't studied this seek business so I have no opinion on that and
what a good size would be, but interesting sizes might be
rounded to both PG_IO_ALIGN_SIZE and filesystem block size according
to fstat(fileno(stream)).  IDK, just a thought...