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

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

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
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