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From: Greg Smith <gsmith@gregsmith.com>
To: James Mansion <james@mansionfamily.plus.com>
Cc: Scott Carey <scott@richrelevance.com>
Cc: Stef Telford <stef@ummon.com>
Cc: Mark Kirkwood <markir@paradise.net.nz>
Cc: pgsql-performance@postgresql.org
Subject: Re: Raid 10 chunksize
Date: Fri, 3 Apr 2009 05:53:25 -0400 (EDT)
Message-ID: <alpine.GSO.2.01.0904030535060.4011@westnet.com> (raw)
In-Reply-To: <49D50F0E.6050807@mansionfamily.plus.com>
References: <C5F933B9.40BB%scott@richrelevance.com>
	<alpine.GSO.2.01.0904020437080.17611@westnet.com>
	<49D50F0E.6050807@mansionfamily.plus.com>

On Thu, 2 Apr 2009, James Mansion wrote:

> Might have to give the whole disk to ZFS with Solaris to give it 
> confidence to enable write cache

Confidence, sure, but not necessarily performance at the same time.  The 
ZFS Kool-Aid gets bitter sometimes too, and I worry that its reputation 
causes people to just trust it when they should be wary. If there's 
anything this thread does, I hope it helps demonstrate how easy it is to 
discover reality doesn't match expectations at all in this very messy 
area.  Trust No One!  Keep Your Laser Handy!

There's a summary of the expected happy ZFS actions at 
http://www.opensolaris.org/jive/thread.jspa?messageID=19264; and a good 
cautionary tale of unhappy ZFS behavior in this area at 
http://blogs.digitar.com/jjww/2006/12/shenanigans-with-zfs-flushing-and-intelligent-arrays/ 
and its follow-up 
http://blogs.digitar.com/jjww/2007/10/back-in-the-sandbox-zfs-flushing-shenanigans-revisted/

Systems with a hardware write cache are pretty common on this list, which 
makes the situation described there not that unlikely to run into.  The 
official word here is at

http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide#FLUSH

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD



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