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.
--
Jim Nasby jim@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)