Message-ID: <4CC4672D.9040401@2ndquadrant.com>
Date: Sun, 24 Oct 2010 13:04:45 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Jesper Krogh <jesper@krogh.cc>
CC: Tom Lane <tgl@sss.pgh.pa.us>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Bruce Momjian <bruce@momjian.us>, jd@commandprompt.com,
	Scott Marlowe <scott.marlowe@gmail.com>,
	Steve Crawford <scrawford@pinpointresearch.com>,
	pgsql-performance@postgresql.org, Ben Chobot <bench@silentmedia.com>
Subject: Re: BBU Cache vs. spindles
References: <4CC04EA50200002500036C5E@gw.wicourts.gov>
	<201010211935.o9LJZTM21744@momjian.us>
	<4CC0513E0200002500036C68@gw.wicourts.gov>
	<4CC1094C.1090306@2ndquadrant.com> <13038.1287720327@sss.pgh.pa.us>
	<4CC1AFB3.3000903@2ndquadrant.com> <4CC1BD86.8070006@krogh.cc>
In-Reply-To: <4CC1BD86.8070006@krogh.cc>
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Jesper Krogh wrote:
> Can you point to some ZFS docs that tell that this is the case..  I'd 
> be surprised
> if it doesnt copy away the old block and replaces it with the new one 
> in-place. The
> other behaviour would quite quickly lead to a hugely fragmented 
> filesystem that
> performs next to useless and ZFS doesnt seem to be in that category..

http://all-unix.blogspot.com/2007/03/zfs-cow-and-relate-features.html

"Blocks containing active data are never overwritten in place; instead, 
a new block is allocated, modified data is written to it, and then any 
metadata blocks referencing it are similarly read, reallocated, and 
written."

http://opensolaris.org/jive/thread.jspa?messageID=19264 discusses how 
this interacts with the common types of hardware around:  no guaratees 
with lying hard drives as always, but otherwise you're fine.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books