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To: James Bottomley <James.Bottomley@suse.de>
Cc: Arjan van de Ven <arjan@infradead.org>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Chris Worley <worleys@gmail.com>,
       Matthew Wilcox <matthew@wil.cx>, Bryan Donlan <bdonlan@gmail.com>,
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       Markus Trippelsdorf <markus@trippelsdorf.de>,
       Matthew Wilcox <willy@linux.intel.com>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>,
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       Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, linux-scsi@vger.kernel.org,
       linux-ide@vger.kernel.org, Linux RAID <linux-raid@vger.kernel.org>
Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap
 slot is freed)
References: <200908122007.43522.ngupta@vflare.org>	 <20090813151312.GA13559@linux.intel.com>	 <20090813162621.GB1915@phenom2.trippelsdorf.de>	 <alpine.DEB.1.10.0908130931400.28013@asgard.lang.hm>	 <87f94c370908131115r680a7523w3cdbc78b9e82373c@mail.gmail.com>	 <alpine.DEB.1.10.0908131342460.28013@asgard.lang.hm>	 <3e8340490908131354q167840fcv124ec56c92bbb830@mail.gmail.com>	 <4A85E0DC.9040101@rtr.ca>	 <f3177b9e0908141621j15ea96c0s26124d03fc2b0acf@mail.gmail.com>	 <20090814234539.GE27148@parisc-linux.org>	 <f3177b9e0908141719s658dc79eye92ab46558a97260@mail.gmail.com>	 <1250341176.4159.2.camel@mulgrave.site> <4A86B69C.7090001@rtr.ca>	 <1250344518.4159.4.camel@mulgrave.site>	 <20090816150530.2bae6d1f@lxorguk.ukuu.org.uk>	 <20090816083434.2ce69859@infradead.org> <1250437927.3856.119.camel@mulgrave.site>
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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.