Configuring disk cache size on postgress

[Dror Matalon]

Hi Folks,

When configuring postgres, one of the variables to configure is
effective_cache_size:
	Sets the optimizer's assumption about the effective size of the disk
	cache (that is, the portion of the kernel's disk cache that will be
	used for PostgreSQL data files). This is measured in disk pages, which
	are normally 8 kB each.
	(http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html)

The conventional wisdom on the postgres list has been that for freebsd
you calculate this by doing `sysctl -n vfs.hibufspace` / 8192).

Now I'm running 4.9 with 2 Gig of ram and sysctl -n vfs.hibufspace
indicates usage of 200MB.

Questions:
1. How much RAM is freebsd using for *disk* caching? Is it part of the
general VM or is it limited to the above 200MB? I read Matt Dillon's
http://www.daemonnews.org/200001/freebsd_vm.html, but most of the
discussion there seems to be focused on caching programs and program
data.

2. Can I tell, and if so how,  how much memory the OS is using for disk
caching?

3. What are the bufspace variables for?

This subject has been touched on before in 
http://unix.derkeiler.com/Mailing-Lists/FreeBSD/performance/2003-09/0045.html
which point to a patch to increase the bufspace, but these messages
don't quite answer my questions.


Regards,

Dror


-- 
Dror Matalon
Zapatec Inc 
1700 MLK Way
Berkeley, CA 94709
http://www.fastbuzz.com
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