Re: [PATCH 00/33] Adaptive read-ahead V12

Wu Fengguang
2006-05-26T01:19:43Z

On Thu, May 25, 2006 at 08:44:15AM -0700, Andrew Morton wrote:
> These are nice-looking numbers, but one wonders. If optimising readahead
> makes this much difference to postgresql performance then postgresql should
> be doing the readahead itself, rather than relying upon the kernel's
> ability to guess what the application will be doing in the future. Because
> surely the database can do a better job of that than the kernel.
>
> That would involve using posix_fadvise(POSIX_FADV_RANDOM) to disable kernel
> readahead and then using posix_fadvise(POSIX_FADV_WILLNEED) to launch
> application-level readahead.
>
> Has this been considered or attempted?

There has been many lengthy debates in the postgresql mailing list,
and it seems that there has been _strong_ resistance to it.

IMHO, a best scheme would be
- leave _obvious_ patterns to the kernel
i.e. all kinds of (semi-)sequential reads
- do fadvise() for _non-obvious_ patterns on _critical_ points
i.e. the index scans

Wu