[geeks] CF as IDE

[tom at borton.info]

Sridhar Ayengar wrote:
>> Actually, I thought a properly adapted and configured CF card _was_ an 
>> SSD.
> 
> Yeah, same difference.
> 
> Peace...  Sridhar

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.

I ended up buying a DMA-capable CF cards for a few hundred dollars when 
I did this about 3 years ago - the environment we were in would not 
allow for the heat of a normal hard drive.

Tom