[ClusterLabs] Wtrlt: Antw: Re: Antw: Re: how important would you consider to have two independent fencing device for each node ?
kwenning at redhat.com
Fri Apr 21 05:24:00 EDT 2017
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
>>>> the read task does hang for a long time, the watchdog itself triggers a
>>>> (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
> * 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. 
As strongly recommended as fencing in general I would say ;-)
> : We (as in SUSE) require use of a watchdog for supported
> configurations, but technically it is optional.
Senior Software Engineer, EMEA ENG Openstack Infrastructure
kwenning at redhat.com
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