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WATCHDOG(4)               OpenBSD Programmer's Manual              WATCHDOG(4)

     watchdog - hardware timers/counters for quick crash recovery

     Hardware watchdog timers are devices that reboot the machine when it
     hangs.  The kernel continually resets the watchdog clock on a regular ba-
     sis.  Thus, if the kernel halts, the clock will time out and reset the
     machine.  Some watchdog timers may be configured to be reset from user-
     land, causing a reboot if process scheduling fails.

     A few watchdog drivers are supported, and most support timeout configura-
     tion via sysctl(8) under the kern.watchdog name:

           kern.watchdog.auto    Automatically reset (`tickle') the watchdog

           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.

     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

     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.

     ddb(4), elansc(4), geodesc(4), wdt(4), config(8), sysctl(8)

     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 de-
     vice is used; the first discovered by the kernel is selected.

OpenBSD 3.8                    December 16, 2004                             1