These probes are most useful when looking into the structures
they provide, which are listed in io.d. For example:
dtrace -n 'io:genunix::start { printf("%d\n", args[0]->bio_bcount); }'
Note that the I/O systems in FreeBSD and Solaris/Illumos are sufficiently
different that there is not a 1:1 mapping from scripts that work
with one to the other.
MFC after: 1 month
