Re: Visibility map and freezing

On Tue, 2009-01-20 at 21:51 +0200, Heikki Linnakangas wrote:

> Attached is a simple patch to only start skipping pages after 20 
> consecutive pages marked as visible in the visibility map. This doesn't 
> do any "look-ahead", so it will always scan the first 20 pages of a 
> table before it starts to skip pages, and whenever there's even one page 
> that needs vacuuming, the next 19 pages will also be vacuumed.
> 
> We could adjust that figure 20 according to table size. Or by 
> seq_page_cost/random_page_cost. But I'm leaning towards a simple 
> hard-coded value for now.

Yes, sounds good. Can we stick to multiples of 2 as the OS readahead
does IIRC? So either 16 or 32. I'd go 32.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support