usb 3.0 thumb drive speed limit

[Gary Palmer]

On Mon, Jan 02, 2017 at 11:27:49AM +0300, Marat N.Afanasyev wrote:
> I wonder is there a speed limit on usb 3.0? I've bought
> 
> ugen0.4: <SanDisk> at usbus0
> umass2 on uhub7
> umass2: <SanDisk Extreme, class 0/0, rev 2.10/0.10, addr 4> on usbus0
> da2 at umass-sim2 bus 2 scbus9 target 0 lun 0
> da2: <SanDisk Extreme 0001> Removable Direct Access SPC-4 SCSI device
> da2: Serial Number AA010808161609220143
> da2: 40.000MB/s transfers
> da2: 59840MB (122552320 512 byte sectors)
> da2: quirks=0x2<NO_6_BYTE>
> 
> that claims 'Up to 245 MBytes/sec read speed'

I don't think the speed reported by the SCSI layer (CAM) is correct
for USB, although it seems from an experiment here that if you 
plug a USB3 drive into a USB2 port it reports "40.000MB/s transfers"
and on a USB3 port it reports "400.000MB/s transfers".  SCSI doesn't
really have any direct mapping to the USB speeds so I suspect the
USB stack uses the closest value.  The tests I did were with 10.3.
Other releases may behave differently.

Check with usbconfig what the negotiated USB speed is (the "spd=" value
with the Mbps or Gbps value in brackets afterwards)

FULL = USB1
HIGH = USB2
SUPER = USB3

Regards,

Gary