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

[Jesper Hansen]

Okay, this sounds very fast indeed. I look forward to hear about your 
experiences.

/Jesper

Aaron Burt wrote:
> 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.
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