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WATCHDOG(4)           DragonFly 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
     desirable 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, leading to the machine being rebooted.  Note that the kernel will
     not automatically 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 options DDB_UNATTENDED to the
     kernel configuration file 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), 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.

DragonFly 5.7-DEVELOPMENT       October 9, 2009      DragonFly 5.7-DEVELOPMENT

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