... whilst I'm here, is this supposed to work?
[root at darkwatch] /data/blockd# dtrace -s test2.d -p 28686
dtrace: script 'test2.d' matched 1 probe
^C
blockd
value ------------- Distribution ------------- count
-1 | 0
0 | 4676
1 |@ 23715
2 | 7642
4 | 9457
8 |@ 32601
16 | 17893
32 | 8685
64 | 1102
128 | 1315
256 | 0
512 | 0
1024 | 0
2048 | 0
4096 | 0
8192 | 0
16384 | 0
32768 | 0
65536 | 0
131072 | 0
262144 | 0
524288 | 0
1048576 | 0
2097152 | 0
4194304 | 0
8388608 | 0
16777216 | 0
33554432 | 0
67108864 | 0
134217728 | 0
268435456 | 0
536870912 | 0
1073741824 | 0
2147483648 | 0
4294967296 | 0
8589934592 | 0
17179869184 | 0
34359738368 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1335242
68719476736 | 0
137438953472 | 0
274877906944 | 0
549755813888 | 0
1099511627776 | 0
2199023255552 | 0
4398046511104 | 0
8796093022208 | 0
17592186044416 | 0
35184372088832 | 0
70368744177664 | 2630
140737488355328 | 0
[root at darkwatch] /data/blockd# cat test2.d
pid$target:libc*:memcpy:return
{
@[execname] = quantize(arg2);
}