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2006-11-24, 08:43 #1
- Join Date
- Feb 2005
- PHL, Pennsylvania, USA
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I needed to transfer a couple of image files from one USB2 drive to
another. So, I decide to time the transfers - with the following results
for two files:
#1 ~6 GB 14 min. 7.54 MBps
#2 ~20 GB 44 min. 7.48 MBps
Quoted transfer speed for USB 2.0 is 480 Mbps or 60 MBps, as you all know.
So, again a demonstration of just how far off real results are from
Anyone have any idea how the spec of 480 Mbps is actually determined?
2006-11-24, 10:09 #2
- Join Date
- Nov 2001
- Thanked 7 Times in 7 Posts
Transferring files from one device to another will almost certainly involve moving the data to memory, one "chunk" at a time, and then moving it to the other device one chunk at a time. (where chunk is some unit of disk usage). Depending on how big these data transfers are, you may be looking at latency issues, rather than bandwidth ones.
For a start you should probably double the 7.5 MBps figure, as the data has been transferred over the USB twice. You then need to realise that there was quite a lot of time when the USB wasn't busy - as the copy software was thinking, checking, planning etc.
Bandwidth figures that vendors quote for any device (or bus) represent a number that they guarantee you won't be able to exceed - it is a theoretical maximum when everything else has been optimised out of the way to leave just a raw transfer of data with no overheads (and probably no useful work being done).
2006-11-24, 12:36 #3
- Join Date
- Jan 2001
- Quedgeley, Gloucester, England
- Thanked 1 Time in 1 Post
The more important value is not the theoretical maximum speed of the USB 2.0 interface, but the speed at which the drives can send and receive data!
For example, just taking the write speed of a USB Flash Drive. a large Corsair Voyager is one of the faster USB Flash Drives around, but the maximum "write" speed is 12 MB/sec. The Cucial Gizmo is quoted at 13 MB/s. Cheaper ones are usually slower. The "read" speeds are usually faster, perhaps 19 MB/s.
So your data transfer rate is probably limited by the speed at which your USB drive can accept data.
If you want to see a spectacular disparity between "theoretical maximum" data transfer rates vs. "actual", have a look at tests of wireless hardware!
John<font face="Script MT Bold"><font color=blue><big><big>John</big></big></font color=blue></font face=script>
Ita, esto, quidcumque...