xl block-attach - cool!

Just wanted to copy something from usb flash drive inside my vm.
Wondered what's the fastest and easiest way to do it and viola:

#> xl block-attach 10 phy:/dev/sdb1 xvdf1 w

then, inside vm mount /dev/xvdf1

afterwards detach it (after freeing resources):

># xl block-detach 10 xvdf1

The interesting thing is that the I got about 20MB/s speed from the usb (it is usually that fast - 3.0 usb drive, plugged in 2.0 port).