MMC quirks relating to performance/lifetime.

[Pavel Machek]

Hi!

> > > I'm not sure if this is the best place to bring this up, but Russel's
> > > name is on a fair share of drivers/mmc code, and there does seem to be
> > > quite a bit of MMC-related discussions. Excuse me in advance if this
> > > isn't the right forum :-).
> > > 
> > > Certain MMC vendors (maybe even quite a bit of them) use a pretty
> > > rigid buffering scheme when it comes to handling writes. There is
> > > usually a buffer A for random accesses, and a buffer B for sequential
> > > accesses. For certain Toshiba parts, it looks like buffer A is 8KB
> > > wide, with buffer B being 4MB wide, and all accesses larger than 8KB
> > > effectively equating to 4MB accesses. Worse, consecutive small (8k)
> > > writes are treated as one large sequential access, once again ending
> > > up in buffer B, thus necessitating out-of-order writing to work around
> > > this.
> > 
> > Hmmmm, I somehow assumed MMCs would be much more cleverr than this.
> 
> No, these devices are incredibly stupid, or extremely optimized to
> a specific use case (writing large video files to FAT32), depending on how
> you look at them.
> 
> > > reorders) them? The thresholds would then be adjustable as
> > > module/kernel parameters based on manfid. I'm asking because I have a
> > > patch now, but its ugly and hardcoded against a specific manufacturer.
> > 
> > How big is performance difference?
> 
> Several orders of magnitude. It is very easy to get a card that can write
> 12 MB/s into a case where it writes no more than 30 KB/s, doing only
> things that happen frequently with ext3.

Ungood.

I guess we should create something like loopback device, which knows
about flash specifics, and does the right coalescing so that card
stays in the fast mode?

...or, do we need to create new, simple filesystem with layout similar
to fat32, for use on mmc cards?
								Pavel
-- 
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