Manipulating disk cache (buf) settings

Vivek Khera vivek at
Mon May 23 08:47:57 PDT 2005

On May 23, 2005, at 10:58 AM, Sven Willenberger wrote:

> We are running a PostgreSQL server (8.0.3) on a dual opteron system  
> with
> 8G of RAM. If I interpret top and vfs.hibufspace correctly (which show
> values of 215MB and 225771520 (which equals 215MB) respectively. My
> understanding from having searched the archives is that this is the
> value that is used by the system/kernel in determining how much disk
> data to cache.

This is correct, from what I understand.  If you take the  
vfs.hibufspace and divide by the page size for postgres (normally  
8192) you get the proper value for the postgres tunable  

However, the value you see is also the max FreeBSD will use without  
hacking up the kernel sources.  I asked about this a while back and  
got a response on what to hack, but I hate keeping local patches to  
the core system which often tend to be forgotten on upgrades...

But I would also love to see the max cache get bigger, especially  
with multi-gig servers becoming more common and affordable.  This  
will kill us on benchmark comparisons for large databases for sure.

Vivek Khera, Ph.D.
+1-301-869-4449 x806

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