Wheres the bottelneck

[tboy55]

Hi.

I'm putting together a bit of a new server and was copying some data over to it.

Running windows 7 , dual xeon 3.4ghz , 2 gig ram computer

I have a 160gig WD sata drive in a stander USB encloser plugged into a USB 2.0 port on this computer
I was copying data from that drive to a SCSI raid 0 array. on IBM u320 10k drives

Windows 7 is telling me the transfer speed is 30MB/s ( 28.7 to 30.1MB/s) it seams to jump around

I am wondering why its not going much faster. itsn usb 2.0 60MB/s and I know the Scsi hard drives can go much faster and the WD 160GB harddrive shouldnt be the slow point either.

Just wondering...

Thanks

 

[nordlead]

It is the USB2.0 interface. While it is technically a 480Mbps interface you'll never sustain such rates and you have to account for overhead.

 

[tboy55]

yep its usb 2.0 both on the external case and the port on the computer
I know 480Mbps is not going to happen by why am I getting like 1/2 of that. is that normal? I have never transfer this much data before.

Thanks

 

[cuecuemore]

Yep, from my experience, 30 MB/s is about the max on many USB 2.0 devices. The bus can handle more, but many thumb drives/USB externals top out at about that same point. I don't know all the reasons for this, and it seems that you can get newer USB 2.0 externals that will do faster I/O.

 

[nordlead]

Yea, it is normal and it has everything to do with the interface. Like I said you never get the actual max (so you might realistically be transferring at 300Mbps), and there is overhead associated with the messages.

That 480Mbps transfer rate has to include the messages with headers and checksums. The data is packed in those message so for every 512 bytes worth of hdd data you might have 20 bytes worth of headers and checksums (making up numbers, but you get the idea). Those extra bytes slow down the effective transfer rate for your HDD data.

If you pulled the HDD from the external case you could probably hit up to 100MB/s.

EDIT: forgot to add that there is also downtime between messages and I believe USB uses "ACK" messages to acknowledge that it received the contents. Those ACK messages would also waste some of the bandwidth.

 

[tboy55]

Ah ok makes sense. I"ll keep 30mb/s in my head as the next max
Thanks for the info