Manipulating disk cache (buf) settings

[Vivek Khera]

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  
effective_cache_size.

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