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

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Message-ID: <4A8834B6.2070104@rtr.ca>
Date:	Sun, 16 Aug 2009 12:32:54 -0400
From:	Mark Lord <liml@....ca>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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)

James Bottomley wrote:
>
> For SSDs, the FTL has to have a separate operation: erase.  Now, one
> could see the correct implementation simply moving the sectors from the
> in-use list to the to be cleaned list and still do the cleaning in the
> background: that would be constant cost (but, again, likely expensive).
> Of course, if SSD vendors decided to erase on the spot when seeing TRIM,
> this wouldn't be true ...
..

The SSDs based upon the Indilinx Barefoot controller appear to do
the erase on the spot, along with a fair amount of garbage collection.
The overhead does vary by size of the TRIM operation (number of sectors
and extents), but even a single-sector TRIM has very high overhead.

Samsung also now has SSDs at retail with TRIM.
I don't have one of those here.
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