Subject: Re: a question about Direct I/O and double buffering
From: Mark Lewis <mark.lewis@mir3.com>
To: Erik Jones <erik@myemma.com>
Cc: Xiaoning Ding <dingxn@cse.ohio-state.edu>,
 pgsql-performance@postgresql.org
In-Reply-To: <7B476592-606F-46F0-A643-9C4E5D85CE6E@myemma.com>
References: <46152D5D.5030602@cse.ohio-state.edu>
	<4C4C834E-9304-4E23-AD15-1756921500B5@myemma.com>
	<1175797629.5581.39.camel@archimedes>
	<7B476592-606F-46F0-A643-9C4E5D85CE6E@myemma.com>
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
Organization: MIR3, Inc.
Date: Thu, 05 Apr 2007 12:56:12 -0700
Message-Id: <1175802972.5581.56.camel@archimedes>
Mime-Version: 1.0

...
[snipped for brevity]
...
> 
> > Not to hijack this thread, but has anybody here tested the behavior
> > of
> > PG on a file system with OS-level caching disabled via forcedirectio
> > or
> > by using an inherently non-caching file system such as ocfs2?
> > 
> > 
> > I've been thinking about trying this setup to avoid double-caching
> > now
> > that the 8.x series scales shared buffers better, but I figured I'd
> > ask
> > first if anybody here had experience with similar configurations.
> > 
> > 
> > -- Mark
> 
> 
> Rather than repeat everything that was said just last week, I'll point
> out that we just had a pretty decent discusson on this last week that
> I started, so check the archives.  In summary though, if you have a
> high io transaction load with a db where the average size of your
> "working set" of data doesn't fit in memory with room to spare, then
> direct io can be a huge plus, otherwise you probably won't see much of
> a difference.  I have yet to hear of anybody actually seeing any
> degradation in the db performance from it.  In addition, while it
> doesn't bother me, I'd watch the top posting as some people get pretty
> religious about (I moved your comments down).

I saw the thread, but my understanding from reading through it was that
you never fully tracked down the cause of the factor of 10 write volume
mismatch, so I pretty much wrote it off as a data point for
forcedirectio because of the unknowns.  Did you ever figure out the
cause of that?

-- Mark Lewis