[En-Nut-Discussion] How to format and use dataflash with RW file system...

[Remi Bilodeau]

Ok,

For my need, wear levelling is not the priority, I write to log file, so
file become bigger and filling the 16 Megabit of my dataflash can take one
or two year....

http://elm-chan.org/fsw/ff/00index_e.html

I check the ELM file system and the formatting fonction is here, fs_mkfs. I
will take Nut/OS AT45 Dataflash driver with ELM library to format my 16mbit
dataflash as FAT12 superfloppy.

More testing to come...


2010/1/26 Harald Kipp <harald.kipp at egnite.de>

> Ole Reinhardt wrote:
>
> > A simple flash file system with ware leveling would be a real cool
> > extension to NutOS :)
>
> Indeed. Some time ago I spent more than a week to evaluate some ideas on
> this topic. Gosh! The required bookkeeping is even more complicated than
> the PHAT stuff.
>
> In general most of the wear leveling file systems are based on
> journaling. Modified sectors are not written back to their old location.
> Instead a new sector is allocated for each update. Somehow you need to
> mark the old sector outdated. During runtime you can use a table in RAM.
> But somehow this table must be build again when rebooting the system.
>
> An important thing to note is, that not the write access but the erase
> outwears the flash. One funny method was to re-program the remaining 1s
> in the first bytes of outdated sectors to zero. However, not all chips
> like this second programming cycle without an initial erase cycle.
>
> Another way is to add a revision number to each sector. During boot-up
> the driver can locate outdated sectors by verifying these numbers.
> Nevertheless, more tables are required to map hardware sectors to files.
>  The standard rule applies here: Fast access requires more RAM.
> Supporting a 2 Gig flash with an 8-bit AVR will become a challenge.
>
> FAT works very well on systems with scarce memory resources. The problem
> with FAT is, that each write access modifies the FAT table, which is
> stored in fixed sectors. These sectors will wear out soon. If you use it
> mainly for read access, it's a good, existing solution.
>
> AFAIK, MMC/SD-Cards map the whole file system into a kind of journaling
> FS, but all known techniques are protected by patents.
>
> From my experience, adding a journal file system to Nut/OS can be done.
> But its implementation is anything else but trivial. If I remember
> correctly, it took more than a year to get the PHAT drivers working
> reliable.
>
> Harald
>
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