Re: Utilizing "direct writes" Re: File system performance and pg_xlog

* Marko Kreen <marko(at)l-t(dot)ee> [010506 03:33] wrote:
> On Sat, May 05, 2001 at 07:01:35PM -0700, Alfred Perlstein wrote:
> > * Marko Kreen <marko(at)l-t(dot)ee> [010505 17:39] wrote:
> > > * double-buffering and incompatibilities of avoiding that
> >
> > Depends on the OS, most Operating systems like FreeBSD and Solaris
> > offer character device access, this means that the OS will DMA
> > directly from the process's address space. Avoiding the double
> > copy is trivial except that one must align and size writes correctly,
> > generally on 512 byte boundries and in 512 byte increments.
>
> PostgreSQL must then also think about write ordering very hard,
> atm this OS business.

Depends. :)

>
> > > * the speed difference will not be very big. Remeber: it _was_
> > > big on OS'es and fs' in year 1990. Today's fs are lot of
> > > better and there should be a os/fs combo that is 95% perfect.
> >
> > Well, here's an idea, has anyone tried using the "direct write"
> > interface that some OS's offer? I doubt FreeBSD does, but I'm
> > positive that Solaris offers it as well as possibly IRIX.
>
> And how much it differs from using FAT? Thats the point I
> want to make. There should be already a fs that is 90% close
> that.

Using FAT is totally up to the vendor's FAT implementation.
Solaris FAT will cache data for a file as long as it's open
which sort of defeats the purpose. Maybe Linux's caching
methods are less effective or have less overhead making FAT
under Linux a win.

One of the problems is that I don't think most vendors consider
thier FAT implementation to be "mission critical", it's possible
that bugs may be present.

Does anyone have that test suite that was just mentioned for
benching Postgresql? (I'd like to try FreeBSD FAT).

--
-Alfred Perlstein - [alfred(at)freebsd(dot)org]
Instead of asking why a piece of software is using "1970s technology,"
start asking why software is ignoring 30 years of accumulated wisdom.