linux-kernel - Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

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Message-ID: <20090814234539.GE27148@parisc-linux.org>
Date:	Fri, 14 Aug 2009 17:45:39 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Chris Worley <worleys@...il.com>
Cc:	Mark Lord <liml@....ca>, Bryan Donlan <bdonlan@...il.com>,
	david@...g.hm, Greg Freemyer <greg.freemyer@...il.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Nitin Gupta <ngupta@...are.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	Linux RAID <linux-raid@...r.kernel.org>
Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap
	slot is freed)

On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote:
> Sooner is better than waiting to coalesce.  The longer an LBA is
> inactive, the better for any management scheme.  If you wait until
> it's reused, you might as well forgo the advantages of TRIM/UNMAP.  If
> a the controller wants to coalesce, let it coalesce.

I'm sorry, you're wrong.  There is a tradeoff point, and it's different
for each drive model.  Sending down a steady stream of tiny TRIMs is
going to give terrible performance.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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