I/O block usage

Command: sar -b

      bread/s
	     Total amount of data read from the devices in blocks  per
	     second.   Blocks  are equivalent to sectors with 2.4 ker-
	     nels and newer and therefore have a size  of  512  bytes.
	     With older kernels, a block is of indeterminate size.

      bwrtn/s
	     Total  amount  of  data  written to devices in blocks per
	     second.