Best disk caching method (and PGSQL performance)

From: Jim C. Nasby (jim_at_nasby.net)
Date: 09/05/03

  • Next message: Sean Chittenden: "Re: Best disk caching method (and PGSQL performance)" 
    Date: Thu, 4 Sep 2003 17:07:09 -0500
To: freebsd-performance@FreeBSD.ORG

    After reading the thread at
    http://www.freebsd.org/cgi/getmsg.cgi?fetch=25060+34014+/usr/local/www/db/text/2003/freebsd-performance/20030727.freebsd-performance
    and the post at
    http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=utf-8&selm=199907290910.CAA06665%40implode.root.com
    I'm a bit confused about how best to get FBSD to cache disk I/O on a
    PostgreSQL server. Reading the VM article at
    http://www.daemonnews.org/200001/freebsd_vm.html has certainly made
    things a bit clearer, but I want to make sure I'm understanding things
    before I go tweaking stuff.

    The inactive queue is only comprised of pages that have been read in as
    mapped memory, correct? I believe PGSQL does not make use of
    memory-mapped I/O, so does that mean that the only place data reads are
    being cached is in the buffer pool? (Note that this contradicts the
    google link).

    If that's indeed the case, then it seems like the only way to get a
    decent amount of data caching is by increasing the buffer size (which
    apparently means increasing kern.nbuf, which also means increasing
    KVA_PAGES (though I'm not at all sure about this).

    If this isn't the case, why have the buffer pool at all? Why not just
    leave buffered disk pages in the inactive or cache queues?

    On another note, does anyone know for certain that PGSQL on FBSD is
    fsyncing only the WAL? I'm seeing a pretty large amount of activity on
    my primary partition and I'm wondering if it's fsyncing everything.
    Also, has anyone played with the other fsync options? 

    -- 
Jim C. Nasby, Database Consultant                  jim@nasby.net
Member: Triangle Fraternity, Sports Car Club of America
Give your computer some brain candy! www.distributed.net Team #1828
Windows: "Where do you want to go today?"
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FreeBSD: "Are you guys coming, or what?"
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  • Next message: Sean Chittenden: "Re: Best disk caching method (and PGSQL performance)"