read/write disk bandwidth monitoring?

[Aditya]

iostat can provide an aggregate number for transactions per second and Bytes
per transaction for a given device, but if it is a block device, is there a
counter accessible from iostat or elsewhere that I can see how much is read
transactions versus write transactions?

> iostat -K -d -I 1
             ad0              ad2              da0            pass0
  KB/t xfrs   MB   KB/t xfrs   MB   KB/t xfrs   MB   KB/t xfrs   MB
  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0.00   0  0.00
  7.50   8  0.06   0.00   0  0.00   0.00   0  0.00   0.00   0  0.00
  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0.00   0  0.00
  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0.00   0  0.00
^C

tells me transactions per device, but not divided into reads versus writes...

Adi