On Sat, 2005-01-01 at 17:01, Bruce Momjian wrote:
> Simon Riggs wrote:
> > Well, I think we're saying: its not in 8.0 now, and we take our time to
> > consider patches for 8.1 and accept the situation that the parameter
> > names/meaning will change in next release.
> I have no problem doing something for 8.0 if we can find something that
> meets all the items I mentioned.
> One idea would be to just remove bgwriter_percent.  Beta/RC users would
> still have it in their postgresql.conf, but it is commented out so it
> should be OK.  If they uncomment it their server would not start but we
> could just tell testers to remove it.  I see that as better than having
> conflicting parameters.

Can't say I like that at first thought. I'll think some more though...

> Another idea is to have bgwriter_percent be the percent of the buffer it
> will scan.  

Hmmm....well that was my original suggestion (bg2.patch on 12 Dec)
(...though with a bug, as Neil pointed out)

> We could default that to 50% or 100%, but we then need to
> make sure all beta/RC users update their postgresql.conf with the new
> default because the commented-out default will not be correct.

...we just differ/ed on what the default should be...

> At this point I see these as our only two viable options, aside from
> doing nothing.

> I realize our current behavior requires a full scan of the buffer cache,
> but how often is the bgwriter_maxpages limit met?  If it is not a full
> scan is done anyway, right?  

Well, if you heavy a very heavy read workload then that would be a
problem. I was more worried about concurrency in a heavy write
situation, but I can see your point, and agree.

(Idea #1 still suffers from this, so we should rule it out...)

> It seems the only way to really add
> functionality is to change bgwriter_precent to control how much of the
> buffer is scanned.

OK. I think you've persuaded me on idea #2, if I understand you right:

bgwriter_percent = 50 (default)
bgwriter_maxpages = 100 (default)

percent is the number of shared_buffers we scan, limited by maxpages.

(I'll code it up in a couple of hours when the kids are in bed)

Best Regards, Simon Riggs

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