Slooooow USB key speeds

[Patrick O'Callaghan]

On 02/01/2009 08:36 PM, Kanwar Ranbir Sandhu wrote:
> On Sun, 2009-02-01 at 18:14 -0500, Kanwar Ranbir Sandhu wrote:
>
>> I'm starting to think something is not quite right with the USB key (or
>> in the way it's being detected): I tested transferring files to my PSP,
>> but didn't see any of the same speed issues. I was able to copy large
>> files back and forth without any problems.
>
> More testing with my PSP and the USB stick has revealed large file
> transfers (e.g. 1 GB in size or more) destroy the transfer speed.
> Transfers start up fast (around 20 MBps), and after a few seconds begin
> to drop.
>
> The difference being the Kingston USB stick drops to 1 MBps or less,
> whereas the PSP is able to maintain a speed of around 6.5 MBps.  Both
> devices connect at hi-speed (480 Mbps).
>
> Anyone know what's going on?

Two things (at least):

1) The initial high speed is an illusion caused by system buffering. You 
can see this easily by copying a smallish file, say 1MB. It will finish 
very quickly at an apparently high speed. Now try to unmount the stick. 
The light will flash as it physically moves the data over the USB 
connection.

2) USB sticks vary enormously in write performance, and there's often a 
tradeoff between capacity and speed. There's definitely a tradeoff 
between speed and cost. The theoretical maximum throughput of the USB 
bus is almost entirely irrelevant in this context (large file copies).

There may also be driver issues of course, but the above two factors 
need to be taken into account.

poc