[Linux-cluster] Dependency graphs, order of startup and failover handling

[Eftychios Eftychiou]

Got a few questions regarding RHCS. Would be grateful if someone could
answer them.

1. Fencing in a two node cluster according to the FAQ could lead to each
node trying to fence each other resulting in total cluster blowup. The
suggested solution of that problem was to use serialized fencing devices.
>From my understanding the above scenario can occur because in a two node
environment each node has 1 vote so there is no way to break the tie. In any
case in a total unrelated section it is mentioned that a quorum disk can be
used to break the tie. Well I am a bit confused since in the section it was
mentioned that quorum disk support was initially removed from RHEL4 but then
was added optionally. What is the proper way to create a two node cluster ?
Is quorum disk required if a serialized fence device is used? Will quorum
disks usage be phased out in the future ? If yes how do you plan to address
the issues in a 2 node cluster.

2. I have seen veritas cluster server (VCS)  being configured and it had the
ability to define a dependency graph of the various services available. Is
such a thing possible on RHCS? Let me be more specific. 2 node cluster. Only
one node is active running all services. 2nd node is passive and in case of
failure should take over from node 1.

Application X depends on the existence of network Z
Application Y also depends on Z
Application U, I and O depend on X
Application G depends on nothing directly.

Is is possible to configure the cluster to behave in the following manner
If U or I or O die restart them
If X dies the stop all applications and relocate them
if Y dies then restart only Y
if Z dies Relocate all applications to Node 2
If G dies restart it
Of course if there are multiple false restarts all applications should be
moved to node 2

The above applications/components should be started in the following order
Z or G ( G does not depend on anything )
X or Y
O, U,I

The actual setup is quite more complicated with multiple dependencies. I
know it is possible to build dependencies in the service definition however
I do not see how the above can be accomplished.

Thanks , any feedback would be appreciated.

Regards,
Eftychios Eftychiou
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