Per Process disk read/write statistics in Mac OS X

Since OP specifically asked for disk I/O statistics I'd recommend

sudo fs_usage -f diskio

which focuses only on read/write events, contrary to -f filesys as mentioned in the accepted answer. (Don't know if the diskio option wasn't available back then.)

Since there isn't an answer here about how to do this programatically, here's some more info. You can get some info out of libproc if you can use C/C++/ObjectiveC++. The function proc_pid_rusage gives you a bunch of resource info for a given process, but the ones related to your question are:

struct rusage_info_v3 {
    ...
    uint64_t ri_diskio_bytesread;
    uint64_t ri_diskio_byteswritten;
    ...
};

Sample code:

pid_t pid = 10000;
rusage_info_current rusage;
if (proc_pid_rusage(pid, RUSAGE_INFO_CURRENT, (void **)&rusage) == 0)
{
    cout << rusage.ri_diskio_bytesread << endl;
    cout << rusage.ri_diskio_byteswritten << endl;
}

See <libproc.h> and <sys/resource.h> for more info.

Use iotop (as root), for example:

iotop -C 3 10

But the best way (for me) is:

sudo fs_usage -f filesys