Fwd: Configuring disk cache size on postgress

This apeared on the Freebsd-perfomace list and though people here could help
as well.



----------  Forwarded Message  ----------

Subject: Configuring disk cache size on postgress
Date: March 16, 2004 10:44 am
From: Dror Matalon <dror@zapatec.com>
To: freebsd-performance@freebsd.org

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.

Regards,

Dror


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