This hook measures the number of I/O operations that took place during a benchmark based on /proc/partitions statistics. To use this hook you must have set CONFIG_BLK_STATS when you compiled your kernel. If you are unsure, cat /proc/partitions. If you have rio and wio columns, then you can use this hook, otherwise you need to recompile the kernel.
This hook has two functions. First it can add a new column to Getstats for a specific partition (e.g., "hda2"), you can enable this functionality by setting the environment variable PARTDIFF_LIST to a space separated list of partitions to measure. Second, a list of partitions and the differences of interesting values (those that are non-zero) are printed in a new block named partdiff. This can be useful for analyzing odd results. If you just want this block to be produced set the MEASURE_PARTDIFF environment variable to a non-zero value.