[geeks] Flash wear leveling

[Alois Hammer]

Good to know.  Thanks.

I was guessing there might be a problem with memory controllers vs.
performance.  I noticed that the SDHC Classes (as in, say, Class 6)
merely guarantee write speed onto empty media.

The other thing I'm trying to find out about is whether the SD Card
Association has any plans to push SDHC higher than Class 6.  So far, I
haven't heard even a flicker of information on that -- leaving SD 1.1
king of the hill in write speed, at least until you get into CFv3 or
CFv4 cards, or expensive USB flash drives.

On Fri, Feb 22, 2008 at 4:15 AM, Jochen Kunz <jkunz at unixag-kl.fh-kl.de> wrote:
> On Thu, 21 Feb 2008 17:27:58 -0500
>  "Alois Hammer" <aloishammer at gmail.com> wrote:
>
>  > * My best guess, based on available information, is that if I start
>  > tossing UFS or ext3 or other filesystems onto flash memory, wear
>  > leveling goes out the window.
>  No.
>
>  The controlers do wear leveling at the logical-to-physical block
>  translation level. At work we developed a solution to stream video to
>  CF and SD cards. We tested several file systems (ext2/3, xfs, reiserfs,
>  FAT, ...) and finaly gave up. The cards do internal caching of blocks
>  for the wear leveling in a (from the outside) unpredictable manner.
>  At some times write throughput droped through the flor because the card
>  internal cache was filled and the card controler started to shuffle
>  blocks around and write them to Flash. This is multiplied by the (Linux)
>  kernel internal block buffer cahce. ("Write storm")
>
>  Finaly we gave up and developed an application speciffic log structured
>  file system in userland. It fills the card in a long linear write from
>  the begining to the end and then starts to overwrite from the beginning.
>  I.e. the CF card is used as a circular buffer. Also: Sometimes we got
>  corrupted fliesystems on CF cards. Most likely because of the card
>  internal cache algorithem was buggy... Linear writing is easy to handle
>  for the card and gives best, constantly high throughput.
>
>  Using the card as a general purpose file storage with only occasional
>  writes is no problem.
>  --
>
>
>  tsch|_,
>        Jochen
>
>  Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/
>
>
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