illumos gate

Certain hardware/drivers misbehave in such a way that I/O issued to them hangs for arbitrary amounts of time (ranging from hours to forever). Since ZFS ends up waiting for these I/Os to finish before completing a transaction group the entire pool (and any process trying to do I/O on that pool) ends up hanging. In this situation it would actually be preferable to panic the system since service would return after a reboot and leave us with a crash dump that can be used to diagnose the underlying problem (the alternative being that the system continues to hang until a user notices and reboots it manually, which means we probably don't even know why the system was hung).