[geeks] CF as IDE

[Joshua Boyd]

On Sun, Aug 02, 2009 at 08:53:32AM -0400, tom at borton.info wrote:
> There is a critical difference between them: most off-the-shelf CF cards  
> do not support any form of DMA.  Nor do they or the adapters support any  
> form of command queuing or enough of a cache to maintain a high  
> throughput.  Having run Linux on a Celeron 600MHz laptop, the loss of  
> throughput is noticeable.  File system caching will hide most of this,  
> but during moderate disk writes, I've measured 3.5s delays where the  
> system froze while the kernel finally dumped that cache to disk.
>
> Embedded devices use the CF, if they have one, for either small amounts  
> of configuration data or for the ram-disk file system.  Most require  
> limited writes and can therefore use off-the-shelf cards; the reads are  
> usually on par with a hard drive.

When I did a product that used CF cards, we used PIO only cards, and they never showed us a multi-second delay.  DMA cards were not hundreds of dollars, but 
at least some of the DMA cards then wouldn't support PIO modes, which was 
somewhat useless.  Eventually we found a card in the 1 gig size we needed that 
did PIO and DMA (about 3 years ago I believe), but by then the price on 2 gig IDE header SSDs came down enoughthat we switched to those instead.