[ClusterLabs] setting up SBD_WATCHDOG_TIMEOUT, stonith-timeout and stonith-watchdog-timeout

Klaus Wenninger kwenning at redhat.com
Mon Dec 19 07:37:09 EST 2016

On 12/17/2016 11:55 PM, Jehan-Guillaume de Rorthais wrote:
> On Wed, 14 Dec 2016 14:52:41 +0100
> Klaus Wenninger <kwenning at redhat.com> wrote:
>> On 12/14/2016 01:26 PM, Jehan-Guillaume de Rorthais wrote:
>>> On Thu, 8 Dec 2016 11:47:20 +0100
>>> Jehan-Guillaume de Rorthais <jgdr at dalibo.com> wrote:
>>>> Hello,
>>>> While setting this various parameters, I couldn't find documentation and
>>>> details about them. Bellow some questions.
>>>> Considering the watchdog module used on a server is set up with a 30s timer
>>>> (lets call it the wdt, the "watchdog timer"), how should
>>>> "SBD_WATCHDOG_TIMEOUT", "stonith-timeout" and "stonith-watchdog-timeout" be
>>>> set?
>>>> Here is my thinking so far:
>>>> "SBD_WATCHDOG_TIMEOUT < wdt". The sbd daemon should reset the timer before
>>>> the wdt expire so the server stay alive. Online resources and default
>>>> values are usually "SBD_WATCHDOG_TIMEOUT=5s" and "wdt=30s". But what if
>>>> sbd fails to reset the timer multiple times (eg. because of excessive
>>>> load, swap storm etc)? The server will not reset before
>>>> random*SBD_WATCHDOG_TIMEOUT or wdt, right?   
>> SBD_WATCHDOG_TIMEOUT (e.g. in /etc/sysconfig/sbd) is already the
>> timeout the hardware watchdog is configured to by sbd-daemon.
> Oh, ok, I did not realized sbd was actually setting the hardware watchdog
> timeout itself based on this variable. After some quick search to make sure I
> understand it right, I suppose it is done there?
> https://github.com/ClusterLabs/sbd/blob/172dcd03eaf26503a10a18501aa1b9f30eed7ee2/src/sbd-common.c#L123
>> sbd-daemon is triggering faster - timeout_loop defaults to 1s but
>> is configurable.
>> SBD_WATCHDOG_TIMEOUT (and maybe the loop timeout as well
>> but significantly shorter should be sufficient)
>> has to be configured so that failing to trigger within time means
>> a failure with high enough certainty or the machine showing
>> comparable response-times would anyway violate timing requirements
>> of the services running on itself and in the cluster.
> OK. So I understand now why 5s is fine as a default value then.
>> Have in mind that sbd-daemon defaults to running realtime-scheduled
>> and thus is gonna be more responsive than the usual services
>> on the system. Although you of course have to consider that
>> the watchers (child-processes of sbd that are observing e.g.
>> the block-device(s), corosync, pacemaker_remoted or
>> pacemaker node-health) might be significantly less responsive
>> due to their communication partners.
> I'm not sure yet to understand clearly the mechanism and interactions of sbd
> with other daemons. So far, I understood that Pacemaker/stonithd was able to
> poke sbd to ask it to trigger a node reset through the wd device. I'm very new
> to this area and I still lake of self documentation.

Pacemaker is setting the node unclean which pacemaker-watcher
(one of sbd daemons) sees as it is connected to the cib.
This is why the mechanism is working (sort of - see the discussion
in my pull request in the sbd-repo) on nodes without stonithd as
well (remote-nodes).
If you are running sbd with a block-device there is of course this
way of communication as well between pacemaker and sbd.
(e.g. via fence_sbd fence-agent)
Be aware that there are different levels of support for these
features in the distributions. (RHEL more on the watchdog-side,
SLES more on the block-device side ... roughly as far as I got it)

>>>> "stonith-watchdog-timeout > SBD_WATCHDOG_TIMEOUT". I'm not quite sure what
>>>> is stonith-watchdog-timeout. Is it the maximum time to wait from stonithd
>>>> after it asked for a node fencing before it considers the watchdog was
>>>> actually triggered and the node reseted, even with no confirmation? I
>>>> suppose "stonith-watchdog-timeout" is mostly useful to stonithd, right?  
>> Yes, the time we can assume a node to be killed by the hardware-watchdog...
>> Double the hardware-watchdog-timeout is a good choice.
> OK, thank you
>>>> "stonith-watchdog-timeout < stonith-timeout". I understand the stonith
>>>> action timeout should be at least greater than the wdt so stonithd will
>>>> not raise a timeout before the wdt had a chance to exprire and reset the
>>>> node. Is it right?  
>> stonith-timeout is the cluster-wide-defaut to wait for stonith-devices
>> to carry out their duty. In the sbd-case without a block-device (sbd used
>> for pacemaker to be observed by a hardware-watchdog) it shouldn't
>> play a role.
> I thought self-fencing through sbd/wd was carried by stonithd because of such
> messages in my PoC log files:
>   stonith-ng: notice: unpack_config: Relying on watchdog integration for fencing

see above ... or read as sit still and wait for the watchdog to do the
job ;-)

> That's why I thought "stonith-timeout" might have a role there, as it looks
> like a stonith device then...
> By pure tech interest here, some more input or documentation to read about how
> it works would be really appreciated.
>> When a block-device is being used it guards the
>> communication with the fence-agent communicating with the
>> block-device.
> OK
> Thank you for your help!

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