On Thu, 1 Jun 2006, Don W. Carr wrote:
> Ok, I plan to buy a 2 Gig SD card for data logging, which should take at
> least 6 months to fill at one sample per second while the train is running.
> I did not realize that the SD and CF cards had internal electronics for wear
> leveling and bad block management. What is the best file system to use? I
> would think ext3 might be a good option. Don.
In general any standard journalling file system (ext3, xfs, reiser) is bad
with flash because they do 3 device writes per software write operation...
- write to journal
- do real data update
- rewrite journal record
As you can see the journal area gets a lot of write hits too, hopefully
the wear levelling will help. But remember never use a journalling
filesystem with a floppy :-)
Some useful comments at...
http://groups.google.co.uk/group/linux.kernel/browse_thread/thread/cee15e056e51b9af/f61e5bfc6375ad5c?lnk=st&q=problems+with+journaling+filesystems+for+flash+disks&rnum=1&hl=en#f61e5bfc6375ad5c
Of course there is JFFS, but from what I understand you need better access
to the flash chips than a standard flash card interface gives you.
cheers
>
> On 6/1/06, Jesse Off <> wrote:
> >
> > --- In "Don W. Carr" <> wrote:
> >
> > > We need to create a HOWTO for doing datalogging to flash memory.
> > Even if you
> > > had to buy new SD cards every five years, it might still be cheaper
> > and more
> > > reliable than purchasing a mini hard drive.
> > >
> > > Don.
> >
> > This is exactly what we've found. The super-miniaturized mechanical
> > guts of a microdrive do not lend themselves well to high MTBF. They
> > use a lot of power and run hot being so small. The intended workload
> > has a very small duty-cycle as they are designed to be powered off and
> > idle most of the time.
> >
> > SD and CF have internal bad block management and wear leveling
> > algorithms that go on in the controller of the card itself. Those
> > electronics make traditional filesystems like FAT or EXT2 no so bad on
> > flash. Also, not many people can argue that modern SD or CF running
> > EXT2 or EXT3 is too slow anymore-- the hardware ASICs on the CF/SD
> > cards are quite fast and effective at what they do.
> >
> > JFFS2 and YAFFS couple the bad block management and wear leveling to
> > the filesystem design itself which is partially the reason their scope
> > is limited to a particular type of raw flash chip technology (nand,
> > nor, 2k page nand, etc...). The methodology of a flash translation
> > layer underneath a standard block filesystem and the good algorithms
> > for it are mostly protected from GPL/Linux implementation by a patent
> > minefield and the perceived opportunity for speed and design
> > perfection by coupling the two distinct tasks (wear leveling/bad block
> > management and UNIX filesystem) together in one uber-filesystem.
> >
> > //Jesse Off
> >
> >
> >
> >
> >
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>
> --
> Dr. Don W. Carr
> J. G. Montenegro 2258
> Guadalajara, Mexico
> +52-333-630-0704
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