[ClusterLabs] Wtrlt: Antw: Re: Antw: Re: how important would you consider to have two independent fencing device for each node ?

[Klaus Wenninger]

On 04/21/2017 10:10 AM, Kristoffer Grönlund wrote:
> Ken Gaillot <kgaillot at redhat.com> writes:
>
>>>> I think it works differently: One task periodically reads ist mailbox slot 
>>>> for commands, and once a comment was read, it's executed immediately. Only
>>> if 
>>>> the read task does hang for a long time, the watchdog itself triggers a
>>> reset 
>>>> (as SBD seems dead). So the delay is actually made from the sum of "write 
>>>> delay", "read delay", "command excution".
>> I think you're right when sbd uses shared-storage, but there is a
>> watchdog-only configuration that I believe digimer was referring to.
>>
>> With watchdog-only, the cluster will wait for the value of the
>> stonith-watchdog-timeout property before considering the fencing successful.
> I think there are some important distictions to make, to clarify what
> SBD is and how it works:
>
> * The original SBD model uses shared storage as its fencing mechanism
>   (thus the name Shared-storage based death) - when talking about
>   watchdog-only SBD, a new mode only introduced in a fork of the SBD
>   project, it would probably help avoid confusion to be explicit about
>   that.
>
> * Watchdog-only SBD relies on quorum to avoid split-brain or fence
>   loops, and thus requires at least three nodes or an additional qdevice
>   node. This is my understanding, correct me if I am wrong. Also, this
>   disqualifies watchdog-sbd from any of Digimers setups since they are
>   2-node only, so that's probably something to be aware of in this
>   discussion. ;)

There is no way around the '2 physical devices are not enough
for an HA-cluster' paradigm ;-)
And Watchdog-only SBD isn't one either although it helps you
to make three nodes without any additional physical device
(stonith-device, disk, ...) work as a reliable cluster.

>
> * The watchdog fencing in SBD is not the primary fence mechanism when
>   shared storage is available. In fact, it is an optional although
>   strongly recommended component. [1]

As strongly recommended as fencing in general I would say ;-)

Regards,
Klaus

> [1]: We (as in SUSE) require use of a watchdog for supported
> configurations, but technically it is optional.
>


-- 
Klaus Wenninger

Senior Software Engineer, EMEA ENG Openstack Infrastructure

Red Hat

kwenning at redhat.com