On Tue, Jun 21, 2022 at 10:33 PM Jakub Wartak <Jakub.Wartak@tomtom.com> wrote:
> > > Maybe the important question is why would be readahead mechanism be
> > disabled in the first place via /sys | blockdev ?
> >
> > Because database should know better than OS which data needs to be
> > prefetched and which should not. Big OS readahead affects index scan
> > performance.
>
> OK fair point, however the patch here is adding 1 syscall per XLOG_BLCKSZ which is not cheap either. The code is
alreadyhot and there is example from the past where syscalls were limiting the performance [1]. Maybe it could be
prefetchingin larger batches (128kB? 1MB? 16MB?) ?
I've always thought we'd want to tell it about the *next* segment
file, to smooth the transition from one file to the next, something
like the attached (not tested).