memory question

PostgreSQL 8.4.3

Linux Redhat 5.0

Question: How much memory do I really need?

From my understanding there are two primary strategies for setting up
PostgreSQL in relationship to memory:

1) Rely on Linux to cache the files. In this approach you set the
shared_buffers to a relatively low number.

2) You can set shared_buffers to a very large percentage of your
memory so that PostgreSQL reserves the memory for the database.

I am currently using option #1. I have 24 Gig of memory on my server
and the database takes up 17 Gig of disk space. When I do the Linux
command "top" I notice that 19 Gig is allocated for cache. Is there a
way for me to tell how much of that cache is associated with the caching
of database files?

I am basically asking how much memory do I really need? Maybe I have
complete over kill. Maybe I am getting to a point where I might need
more memory.

My thought was I could use option #2 and then set the number to a lower
amount. If the performance is bad then slowly work the number up.

Our server manager seems to think that I have way to much memory. He
thinks that we only need 5 Gig. I don't really believe that. But I
want to cover myself. With money tight I don't want to be the person
who is wasting resources. We need to replace our database servers so I
want to do the right thing.

Thanks,

Lance Campbell

Software Architect/DBA/Project Manager

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