Re: Large tables (was: RAID 0 not as fast as

Guy Thornley wrote:
> > >> I thought that posix_fadvise() with POSIX_FADV_WILLNEED was exactly
> > >> meant for this purpose?
> > >
> > > This is a good idea - I wasn't aware that this was possible.
> >
> > This possibility was the reason for me to propose it. :-)
>
> posix_fadvise() features in the TODO list already; I'm not sure if any work
> on it has been done for pg8.2.
>
> Anyway, I understand that POSIX_FADV_DONTNEED on a linux 2.6 kernel allows
> pages to be discarded from memory earlier than usual. This is useful, since
> it means you can prevent your seqscan from nuking the OS cache.
>
> Of course you could argue the OS should be able to detect this, and prevent
> it occuring anyway. I don't know anything about linux's behaviour in this
> area.

We tried posix_fadvise() during the 8.2 development cycle, but had
problems as outlined in a comment in xlog.c:

/*
* posix_fadvise is problematic on many platforms: on older x86 Linux
* it just dumps core, and there are reports of problems on PPC platforms
* as well. The following is therefore disabled for the time being.
* We could consider some kind of configure test to see if it's safe to
* use, but since we lack hard evidence that there's any useful performance
* gain to be had, spending time on that seems unprofitable for now.
*/

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

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