Date: Tue, 14 Oct 2008 15:27:14 -0200 From: "Olivier Gautherot" <olivier@gautherot.net> To: "M. Warner Losh" <imp@bsdimp.com> Cc: freebsd-embedded@freebsd.org Subject: Re: storage selection for embedded devices Message-ID: <dcfb161c0810141027s12219080wb403116c1f562aa@mail.gmail.com> In-Reply-To: <20081014.110022.635731567.imp@bsdimp.com> References: <48F4C02F.1060407@syx.ca> <20081014.110022.635731567.imp@bsdimp.com>
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Philip, Warner, On Tue, Oct 14, 2008 at 3:00 PM, M. Warner Losh <imp@bsdimp.com> wrote: > In message: <48F4C02F.1060407@syx.ca> > Philip Mullis <philip.mullis@syx.ca> writes: > : I was wondering if anyone has extended experience in this area with > : embedded devices. > : > : I have a fixed embedded image which runs happily out of a 1Gig compact > : flash card. > : > : However I have some applications that I want to install to my device > : that will perform writes alot to the cf. > > I've deployed CF cards into systems for a number of years (since > 2000). They are way more reliable than spinning media in the > environments that I deploy my company's gear into. > > We have most of the CF dedicated to a read-only partition. A small > modification partition was also provided. I wonder if you're talking about the same thing (may be just me...) Philip, what frequency of writes are you talking about? I think this is key to the discussion. Are you planning enough RAM to avoid swap? Can your system count with a RAM disk and regular dump of the content to FLASH? If this is the case, a USB stick should be a safe approach. The algorithm Sandisk is referring to enhances the statistical lifespan by shuffling the cells and using spare ones when the main array wears out (trial-and-error algorithm). The typical lifespan of a cell is 100k write cycles: try to evaluate whether this is compatible with the use you plan for your device. > I've also tried to wear out a CF part by writing to the part, both > directly and through a filesystem, millions of times. I was unable to > keep a machine running long enough to cause a failure (my mistake was > doing it in a lab where people liked to unplug things). The technology has surely evolved since I last dealt with it in an industrial environment. However, I would not swear by the "millions of times" as such: Sandisk is famous for leveling the writes over the whole array. Now, if your partition is relatively empty, your device will support more cycles. In any case, using 10% of the FLASH blocks can surely lead you to the millions of cycles without problem. > : Ive read the sandisk wear leveling white paper, yet I also hear many > : people such as professional photographers swearing by the write once > : rule with cf cards. That's paranoia, especially with todays technologies. -- Olivier Gautherot olivier@gautherot.net Cel:+56 98 730 9361 www.gautherot.net http://www.linkedin.com/in/ogautherot
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