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WATCHDOG(4)              BSD Kernel Interfaces Manual              WATCHDOG(4)

NAME
     watchdog -- hardware timers/counters for quick crash recovery

DESCRIPTION
     Hardware watchdog timers are devices that reboot the machine when it
     hangs.  The kernel continually resets the watchdog clock on a regular
     basis.  Thus, if the kernel halts, the clock will time out and reset the
     machine.  Watchdog timers may be configured to be reset from userland to
     cause a reboot if process scheduling fails; see watchdogd(8) for further
     information.

     A number of hardware watchdogs are supported, and all are configured
     using sysctl(8) under the kern.watchdog name:

           kern.watchdog.auto    Automatically reset ('tickle') the watchdog
                                 timer but disable it at system shutdown time.

           kern.watchdog.period  The timeout in seconds.  Setting it to zero
                                 disables the watchdog timer.

     In situations where the machine provides vital services which are not
     handled completely in kernel space, e.g. mail exchange, it may be desir-
     able to reboot the machine if process scheduling fails.  This is done by
     setting kern.watchdog.auto to zero and running a process which repeatedly
     sets kern.watchdog.period to the desired timeout value.  Then, if process
     scheduling fails, the process resetting the timer will not be run, lead-
     ing to the machine being rebooted.  Note that the kernel will not auto-
     matically disable an enabled watchdog at system shutdown time when
     kern.watchdog.auto is set to zero.

     Watchdog timers should be used in high-availability environments where
     getting machines up and running quickly after a crash is more important
     than determining the cause of the crash.  A watchdog timer enables a
     crashed machine to autonomously attempt to recover quickly after a system
     failure.

     Note that this also means that it is unwise to combine watchdog timers
     with ddb(4) since the latter may prevent the former from resetting the
     watchdog timeout before it expires.  This means that the machine will be
     rebooted before any debugging can be done.  In other words: For mission
     critical machines, disable ddb(4) by adding ``ddb.panic=0'' to
     sysctl.conf(5) since this will give the chance to perform a crash dump
     and reboot.  Simply setting the watchdog will lose the debug trace of
     what went wrong.

SEE ALSO
     ddb(4), sysctl.conf(5), config(8), sysctl(8), watchdogd(8)

BUGS
     For systems with multiple watchdog timers available, only a single one
     can be used at a time.  There is currently no way of selecting which
     device is used; the first discovered by the kernel is selected.

BSD                              May 21, 2009                              BSD