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

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Date:	Sat, 15 Aug 2009 08:55:17 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Mark Lord <liml@....ca>
Cc:	Chris Worley <worleys@...il.com>, Matthew Wilcox <matthew@....cx>,
	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 Sat, 2009-08-15 at 09:22 -0400, Mark Lord wrote:
> James Bottomley wrote:
> >
> > This means you have to drain the outstanding NCQ commands (stalling the
> > device) before you can send a TRIM.   If we do this for every discard,
> > the performance impact will be pretty devastating, hence the need to
> > coalesce.  It's nothing really to do with device characteristics, it's
> > an ATA protocol problem.
> ..
> 
> I don't think that's really much of an issue -- we already have to do
> that for cache-flushes whenever barriers are enabled.  Yes it costs,
> but not too much.

That's not really what the enterprise is saying about flush barriers.
True, not all the performance problems are NCQ queue drain, but for a
steady workload they are significant.

James

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