Date: Mon, 16 Aug 2010 23:05:52 +0200
From: Andres Freund <andres@anarazel.de>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Scott Carey <scott@richrelevance.com>, Bruce Momjian <bruce@momjian.us>,
	Kenneth Cox <kenstir@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Advice configuring ServeRAID 8k for performance
Message-ID: <20100816210552.GC3911@anarazel.de>
References: <201008131817.o7DIH1b03550@momjian.us>
	<4C6591D9.1080206@2ndquadrant.com>
	<5687A8AD-CA70-4272-A32C-6AE5F491CEA4@richrelevance.com>
	<4C69796D.9000408@2ndquadrant.com>
	<20100816200227.GA3911@anarazel.de>
	<4C699BE2.50606@2ndquadrant.com>
	<20100816201838.GB3911@anarazel.de>
	<4C69A57B.80605@2ndquadrant.com>
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On Mon, Aug 16, 2010 at 04:54:19PM -0400, Greg Smith wrote:
> Andres Freund wrote:
> >A new checkpointing logic + a new syncing logic
> >(prepare_fsync() earlier and then fsync() later) would be a nice
> >thing. Do you plan to work on that?
> The background writer already caches fsync calls into a queue, so
> the prepare step you're thinking needs to be there is already.  The
> problem is that the actual fsync calls happen in a tight loop.  That
> we're busy fixing.
That doesn't help that much on many systems with a somewhat deep
queue. An fsync() equals a barrier so it has the effect of stopping
reordering around it - especially on systems with larger multi-disk
arrays thats pretty expensive.
You can achieve surprising speedups, at least in my experience, by
forcing the kernel to start writing out pages *without enforcing
barriers* first and then later enforce a barrier to be sure its
actually written out. Which, in a simplified case, turns the earlier
needed multiple barriers into a single one (in practise you want to
call fsync() anyway, but thats not a big problem if its already
written out).

Andres