RE: Use fadvise in wal replay

> > 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?)  ? 

-J.

[1] - https://commitfest.postgresql.org/28/2606/