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: <4C4C834E-9304-4E23-AD15-1756921500B5@myemma.com>
References: <46152D5D.5030602@cse.ohio-state.edu>
	<4C4C834E-9304-4E23-AD15-1756921500B5@myemma.com>
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Organization: MIR3, Inc.
Date: Thu, 05 Apr 2007 11:27:09 -0700
Message-Id: <1175797629.5581.39.camel@archimedes>
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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

On Thu, 2007-04-05 at 13:09 -0500, Erik Jones wrote:
> On Apr 5, 2007, at 12:09 PM, Xiaoning Ding wrote:
> 
> > Hi,
> > 
> > 
> > A page may be double buffered in PG's buffer pool and in OS's buffer
> > cache.
> > Other DBMS like DB2 and Oracle has provided Direct I/O option to
> > eliminate
> > double buffering. I noticed there were discusses on the list. But
> > I can not find similar option in PG. Does PG support direct I/O now?
> > 
> > 
> > The tuning guide of PG usually recommends a small shared buffer pool
> > (compared
> > to the size of physical memory).  I think it is to avoid swapping.
> > If
> > there were
> > swapping, OS kernel may swap out some pages in PG's buffer pool even
> > PG
> > want to keep them in memory. i.e. PG would loose full control over
> > buffer pool.
> > A large buffer pool is not good because it may
> > 1. cause more pages double buffered, and thus decrease the
> > efficiency of
> > buffer
> > cache and buffer pool.
> > 2. may cause swapping.
> > Am I right?
> > 
> > 
> > If PG's buffer pool is small compared with physical memory, can I
> > say
> > that the
> > hit ratio of PG's buffer pool is not so meaningful because most
> > misses
> > can be
> > satisfied by OS Kernel's buffer cache?
> > 
> > 
> > Thanks!
> 
> 
> To the best of my knowledge, Postgres itself does not have a direct IO
> option (although it would be a good addition).  So, in order to use
> direct IO with postgres you'll need to consult your filesystem docs
> for how to set the forcedirectio mount option.  I believe it can be
> set dynamically, but if you want it to be permanent you'll to add it
> to your fstab/vfstab file.
> 
> 
> erik jones <erik@myemma.com>
> software developer
> 615-296-0838
> emma(r)
> 
> 
> 
> 
>