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memory question

From: "Campbell, Lance" <lance(at)illinois(dot)edu>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: memory question
Date: 2010-03-25 00:49:10
Message-ID: (view raw or whole thread)
Lists: pgsql-performance
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.




Lance Campbell

Software Architect/DBA/Project Manager

Web Services at Public Affairs




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