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

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Date:	Fri, 14 Aug 2009 18:19:39 -0600
From:	Chris Worley <worleys@...il.com>
To:	Matthew Wilcox <matthew@....cx>
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 5:45 PM, Matthew Wilcox<matthew@....cx> wrote:
> 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.

Sounds like you might be using junk for a device?

For junk, a little coalescing may be warranted... like in the I/O
schedular, but no more than 100usecs wait before posting, or then you
effect high performing devices too.

Chris
>
> --
> 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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