[En-Nut-Discussion] Nut 3: SD/MMC speed?

[Aaron Burt]

On Fri, Jan 27, 2006 at 10:38:45PM +0100, Jesper Hansen wrote:
> Aaron,
> 
> I think you missed my point. The speed problem it not with the transfer, 
> it's with the block write time of the card.

Indeed.  Luckily, we have digital photographers demanding ever-faster
cards.  And so we have Sandisk CF and SD cards that claim to handle 
20 MB/s sustained writes.  I quite intend to see if their word is good.

> A fast standard card has a block write time of typ 20 mS. Even if your 
> 133x card is 133 times faster it still takes about 150 uS per sector 
> write. That's 300 mS/MB or a max throughput of about 3.3 MB/s.

Ah, sorry, I didn't explain.  Consumer flash cards are rated in
multiples of 150 kB/s, so 133x = 20 MB/s

> Note that this is typical values, max is about 10 times worse, and will 
> hit without warning. This will be seen with a card that has been used 
> for some time. The internal flash wear out, and the wear levelling 
> routines and shifting around of blocks by the internal processor slow 
> down the writes.

I can believe that.  I'll be curious to see how that works with the
super-fast stuff.