Jim C. Nasby wrote:
> On Mon, Jan 22, 2007 at 12:17:39PM -0800, Ron Mayer wrote:
> > Gregory Stark wrote:
> > >
> > > Actually no. A while back I did experiments to see how fast reading a file
> > > sequentially was compared to reading the same file sequentially but skipping
> > > x% of the blocks randomly. The results were surprising (to me) and depressing.
> > > The breakeven point was about 7%. [...]
> > >
> > > The theory online was that as long as you're reading one page from each disk
> > > track you're going to pay the same seek overhead as reading the entire track.
> >
> > Could one take advantage of this observation in designing the DSM?
> >
> > Instead of a separate bit representing every page, having each bit
> > represent 20 or so pages might be a more useful unit. It sounds
> > like the time spent reading would be similar; while the bitmap
> > would be significantly smaller.
>
> If we extended relations by more than one page at a time we'd probably
> have a better shot at the blocks on disk being contiguous and all read
> at the same time by the OS.
Actually, there is evidence that adding only a single page to the end
causes a lot of contention for that last page, and that adding a few
might be better.
-- Bruce Momjian bruce@momjian.us EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +