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Re: DB cache size strategies

From: "Ed L(dot)" <pgsql(at)bluepolka(dot)net>
To: pgsql-general(at)postgresql(dot)org
Subject: Re: DB cache size strategies
Date: 2004-01-30 22:19:56
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On Friday January 30 2004 2:33, Ed L. wrote:
> One key aspect of pgsql performance tuning is to adjust the memory
> consumption settings (shared_buffers, sort_mem, etc) large enough to hold
> as much of the database in shared memory as possible while not causing
> page swap-ins.  I understand that both page swap-outs and swap space
> usage is normal and OK, but lots of page swap-ins are bad).  In other
> words, for absolute fastest performance, we want a database buffer cache
> hit rate of as close to 100% as possible.

I'm also curious about the relationship of DB shared buffer cache to the 
linux/hpux kernel caches.  In particular, if the block being retrieved in 
pgsql was in the kernel's cache but not in the DB cache, thereby forcing a 
read() system call, what kind of quantitative difference in performance 
would one expect when comparing with block retrievals coming from the 
cache?  I would think they'd differ only by something on the order of 
microseconds.  Is the linux kernel disk cache normally a duplicate of much 
of what is in the DB cache?  For linux, does the kernel cache use only 
"available" memory until a program needs it, while the pgsql DB cache 
memory is guaranteed at startup?



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