[Linux-cluster] Fencing required for node failover

[Achint Mehta]

On Thu, Dec 29, 2011 at 5:06 PM, Digimer <linux at alteeve.com> wrote:

> On 12/29/2011 04:49 PM, Achint Mehta wrote:
> > Hi All,
> >
> > I am using RHCS in RHEL 6.2.
> >
> > I am trying to perform a failover for a node in the cluster.
> > All the services have fail-over configured on them with recovery method
> > set to relocate.
> > When the node foes down the services are not relocated to to another
> nodes.
> >
> > Though the node failure is detected by rgmanager:
> > ------
> > Dec 29 16:20:57 rgmanager State change: pcs_linuxha_1 DOWN
> > Dec 29 16:28:25 rgmanager Status Child Max set to 7
> > ------
> > and fenced has the following logs:
> > ------
> > Dec 29 16:21:04 fenced fencing node pcs_linuxha_1
> > Dec 29 16:21:04 fenced fence pcs_linuxha_1 dev 0.0 agent none result:
> > error no method
> > Dec 29 16:21:04 fenced fence pcs_linuxha_1 failed
> > ------
> >
> > 1. Do I require fencing to be enabled to make node failover work
> > 2. If yes, what kind of failover device should I add. (all the nodes are
> > simple servers.)
> >
> >
> > Thanks!
> > Achint
>
> Yes, you absolutely needs fencing.
>
> As soon as a node is lost, fenced informs dlm which then stops providing
> locks. Only when the fence succeeds is dlm informed and will again issue
> locks. In turn, rgmanager uses dlm, so with dlm not providing locks,
> rgmanager can't recover services.
>
> See this for a more specific explanation;
>
>
> https://alteeve.com/w/2-Node_Red_Hat_KVM_Cluster_Tutorial#Concept.3B_Fencing
>
> --
> Digimer
> E-Mail:              digimer at alteeve.com
> Freenode handle:     digimer
> Papers and Projects: http://alteeve.com
> Node Assassin:       http://nodeassassin.org
> "omg my singularity battery is dead again.
> stupid hawking radiation." - epitron
>

Thanks for the explanation.

It appears that until fence daemon performs the fencing activity the
recovery will not be made.
So I would have to setup fencing.

1. I am not sure what kind of fencing device to add for simple servers.
2. Is this fencing device supposed to be a dedicated machine outside of the
cluster.
3. Also, would fencing be successful if the node  to be fenced is not
reachable on the network.

--Achint
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