linux-kernel - Re: swap on eMMC and other flash

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Message-ID: <20120716132935.GA20549@elf.ucw.cz>
Date:	Mon, 16 Jul 2012 15:29:35 +0200
From:	Pavel Machek <pavel@....cz>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnd Bergmann <arnd@...db.de>, linaro-kernel@...ts.linaro.org,
	linux-mm@...ck.org, "Luca Porzio (lporzio)" <lporzio@...ron.com>,
	Alex Lemberg <alex.lemberg@...disk.com>,
	linux-kernel@...r.kernel.org, Saugata Das <saugata.das@...aro.org>,
	Venkatraman S <venkat@...aro.org>,
	Yejin Moon <yejin.moon@...sung.com>,
	Hyojin Jeong <syr.jeong@...sung.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	kernel-team@...roid.com, "Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: swap on eMMC and other flash

On Wed 2012-04-11 13:28:39, Adrian Hunter wrote:
> On 04/04/12 15:47, Arnd Bergmann wrote:
> > On Wednesday 04 April 2012, Adrian Hunter wrote:
> >> On 30/03/12 21:50, Arnd Bergmann wrote:
> >>> (sorry for the duplicated email, this corrects the address of the android
> >>> kernel team, please reply here)
> >>>
> >>> On Friday 30 March 2012, Arnd Bergmann wrote:
> >>>
> >>>  We've had a discussion in the Linaro storage team (Saugata, Venkat and me,
> >>>  with Luca joining in on the discussion) about swapping to flash based media
> >>>  such as eMMC. This is a summary of what we found and what we think should
> >>>  be done. If people agree that this is a good idea, we can start working
> >>>  on it.
> >>
> >> There is mtdswap.
> > 
> > Ah, very interesting. I wasn't aware of that. Obviously we can't directly
> > use it on block devices that have their own garbage collection and wear
> > leveling built into them, but it's interesting to see how this was solved
> > before.
> > 
> > While we could build something similar that remaps blocks between an
> > eMMC device and the logical swap space that is used by the mm code,
> > my feeling is that it would be easier to modify the swap code itself
> > to do the right thing.
> > 
> >> Also the old Nokia N900 had swap to eMMC.
> >>
> >> The last I heard was that swap was considered to be simply too slow on hand
> >> held devices.
> > 
> > That's the part that we want to solve here. It has nothing to do with
> > handheld devices, but more with specific incompatibilities of the
> > block allocation in the swap code vs. what an eMMC device expects
> > to see for fast operation. If you write data in the wrong order on
> > flash devices, you get long delays that you don't get when you do
> > it the right way. The same problem exists for file systems, and is
> > being addressed there as well.
> > 
> >> As systems adopt more RAM, isn't there a decreasing demand for swap?
> > 
> > No. You would never be able to make hibernate work, no matter how much
> > RAM you add ;-)
> 
> Have you considered making hibernate work without swap?

It does work without swap. See userland suspend packages, where you
write the image is up-to you.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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