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 [EMAIL PROTECTED] EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
