Can't reboot or shutdown when an I / O suspending

[root@supool] ~# uname -a

FreeBSD supool.local 8.3-RELEASE-p8 FreeBSD 8.3-RELEASE-p8 #1 r252195M: Mon Aug 18 19:30:12 CST 2014     root@karas.karas:/root/trunk/os-base/amd64/root/trunk/FreeBSD/src/sys/FREENAS.amd64  amd64

[root@supool] /# camcontrol devlist

<SEAGATE ST4000NM0023 MS00>        at scbus2 target 9 lun 0 (da16,pass0)

<SEAGATE ST3450857SS XREC>         at scbus2 target 10 lun 0 (da17,pass1)

<SEAGATE ST31000424SS XRMA>        at scbus2 target 13...