[ClusterLabs] Coming in Pacemaker 2.0.5: on-fail=demote / no-quorum-policy=demote

clusterlabs at t.poki.me clusterlabs at t.poki.me
Thu Aug 13 06:32:32 EDT 2020


wanted to point out one thing that occurred to me when thinking about
the paragraph below.

On 8/12/20 8:57 PM, Ken Gaillot wrote:
> Similarly, Pacemaker offers the cluster-wide "no-quorum-policy" option
> to specify what happens to resources when quorum is lost (the default
> being to stop them). With 2.0.5, "demote" will be a possible value here
> as well, and will mean "demote all promotable resources and stop all
> other resources".
> The intended use case is an application that cannot cause any harm
> after being demoted, and may be useful in a demoted role even if there
> is no quorum. A database that operates read-only when demoted and
> doesn't depend on any non-promotable resources might be an example.

Perhaps not that expected corollary with this cluster-wide setting
(only acknowledged when the cluster is upgraded in its entirety?), if
I understand it correctly, is that previously promoted resource will
get stopped anyway once it depends on a simple resource that doesn't
specify "on-fail" on its own (putting global/resource defaults aside).

It is this implicit resource "composability" (a.k.a. resource trees,
with some tweaks applicable right at this "composed service" level)
idea of long declined RGManager (RIP) that is still quite an appealing
and natural way (despite having less expressive power in general) one
can think of composed services (i.e. the behaviour of a final 	
brings-me-value unit rather than sum of moving parts behind it).
Through the prism of this tree model, if it could be proved that no
other simple resources share dependency on any of the prerequisites
with this promoted resource to be demoted because of "on fail" event,
it would be intuitive to expect they will be kept running and hence
will prevent this promoted resource from consequentially being stopped
despite just a weaker form of its demotion is the first and foremost
choice requested by the user.  Similarly with clones and other
promotable prerequisites, except it might be wise to demote them as
well if it would not conflict with demotion of the dependent promoted
resource that just suffered a failure.

Rationale for this is that prerequisite resource _solely_ consumed
resources that don't need quorum as well hardly needs quorum on its own,
otherwise this is a conflicting/fishy configuration in some way.
(It would likewise be interesting and configuration-proofing-friendly
to investigate such "composing rules of soundness".)

I see there are some practical limits to these semi-recursive and
potentially explosive graph problems, but there's also a question
what's an intuitively expected behaviour, and possible disconnect
could at least be addressed in the documentation.

[Alternatively, some kind of "back-propagation" (intuitively,
"this terminal resource has powers to steer the behaviour of its
prerequisites, as the configuration of this terminal resource is
what's wanted by the outer surroundings of this box, afterall)
flag could be devised such that it would override any behaviour of
the prerequisite resources on a subset of conflicting options, like
on-fail, given that there is no conflict with any other resource
dependent on the same resource.]

Thanks & cheers


More information about the Users mailing list