Re: DB cache size strategies

On Friday January 30 2004 6:06, Martijn van Oosterhout wrote:
> On Fri, Jan 30, 2004 at 03:19:56PM -0700, Ed L. wrote:
> >
> > I'm also curious about the relationship of DB shared buffer cache to
> > the linux/hpux kernel caches.  ...
>
> Whenever the database needs a block not in memory it get loaded and ends
> up in both the OS cache and the DB cache. The difference between getting
> a block out of DB cache and OS cache is very, very small compared to
> loading off disk.

So the OS cache and the DB cache may well have duplicate copies of the data?
With OS cache being more susceptible to preemption from other processes?

> Finally, at least on Linux, the shared memory postgres uses for cache can
> also be swapped out making it very difficult to determine the correct
> value.

What would cause DB shared memory to be swapped out in linux?  I thought it
was all pre-allocated at pgsql startup.  Is this the "well its not really
pre-allocated but rather supplied on demand" story?  I think I saw some
sort of kernel parameter controlling a similar (same?) policy...

TIA.