dan wrote:
> b) USB chipsets must hand off data to the CPU for processing which 
> causes each piece of data to have additional latency going through the 
> CPU once as raw USB packets to be translated by the driver and then 
> again by whatever app is processing that data.  SATA/SAS/IDE all have 
> DMA so they can dump the usable data to memory and the CPU can process 
> it once from there.
> c)because USB packets (for storage devices) are fairly simple packets to 
> decode, its the mhz that matter as its how fast the packet can be pushed 
> through.  Improving controller design can only have a marginal impact on 
> performance unless a high speed controller is used specifically for 
> storage devices(i dont believe there are any on the market).
> d)USB devices rely on a driver to process the raw USB packets into 
> scsi/ide/ata packets.  SATA/IDE controllers require a driver only to 
> read packets already in the appropriate format.  More processing is done 
> in the driver and software tends to have more latency that hardware.

All of the above is incorrect in more ways than I care to go into.

But the core message is the same:  USB *is* slower than eSATA simply 
because USB 2.0 has a maximum bandwidth of 480mbps.  After driver and 
protocol overhead, you usually get about 33 MB/s out of a USB drive. 
The same drive hooked up via eSATA runs at drive speed, usually between 
60MB/s to 120MB/s depending on how fast the drive actually is.
-- 
Jim Leonard (trixter AT oldskool DOT org)            http://www.oldskool.org/
Help our electronic games project:           http://www.mobygames.com/
Or check out some trippy MindCandy at     http://www.mindcandydvd.com/
A child borne of the home computer wars: http://trixter.wordpress.com/

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