We got bitten by this back in the Solaris days in 2009, on a TV broadcasting production box with <i>quite stringent</i> uptime requirements: what happens is the defragmenter gets itself tied in knots and starts thrashing, and the symptom is 50% system CPU with no apparent cause. Got a Sun kernel engineer on call and all. SPOILER: it did in fact require a reboot to unfuck the system. Then we kept the disk in question being wasted.