Dtrace does not work

I want to get some specific I/O data from FreeBSD. IOStat command does not return what I want, so I think about using dtrace. The problem is that I am getting this error while executing dtrace command:
[2.3.1-RELEASE][admin@machine]/usr/share/dtrace: dtrace -s iotime.d
dtrace: failed to initialize dtrace: DTrace device not available on system
[2.3.1-RELEASE][admin@machine]/usr/share/dtrace: which dtrace
[2.3.1-RELEASE][admin@machine]/usr/share/dtrace: dtrace -V
dtrace: Sun D 1.12.1

It seems it is loaded in kernel, but I don't know what should I do to make it works. My FreeBSD info:
[2.3.1-RELEASE][admin@machine]/usr/share/dtrace: uname -a
FreeBSD machine 10.3-RELEASE-p3 FreeBSD 10.3-RELEASE-p3 #2 1988fec(RELENG_2_3_1): Wed May 25 14:14:46 CDT 2016     root@ce23-amd64-builder:/builder/pfsense-231/tmp/obj/builder/pfsense-231/tmp/FreeBSD-src/sys/pfSense  amd64

There is a pfsense loaded in this system (if this information helps).

I can't help much, but I don't understand why SirDice gave the "not freeBSD questions" link

I do know dtrace is a port from Sun Microsystems Solaris (which was branched off BSD earlier). It's not fully supported "yet" (if ever) and questions for it would be a "ported application" questions.

as far as it not worker at all "not supported on system", i have no idea i think read the Wiki page for the dtrace port - i believe there is one
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pFsense is not FreeBSD. It is an appliance which was in the past built on the top of FreeBSD. They also used to have their own forum.