Kevin Tomlinson wrote:
On Tue, 2007-11-06 at 10:25 -0700, Alan Robertson wrote:
We now have the ComponentFail test in CTS. Thanks Lars for getting it
going!
And, in the process, it's showing up some kinds of problems that we
hadn't been looking for before. A couple examples of such problems can
be found here:
http://old.linux-foundation.org/developer_bugzilla/show_bug.cgi?id=1762
http://old.linux-foundation.org/developer_bugzilla/show_bug.cgi?id=1732
The question that comes up is this:
For problems that should "never" happen like death of one of our
core/key processes, is an immediate reboot of the machine the right
recovery technique?
The advantages of such a choice include:
It is fast
It will invoke recovery paths that we exercise a lot in testing
It is MUCH simpler than trying to recover from all these cases,
therefore almost certainly more reliable
The disadvantages of such a choice include:
It is crude, and very annoying
It probably shouldn't be invoked for single-node clusters (?)
It could be criticized as being lazy
It shouldn't be invoked if there is another simple and correct method
Continual rebooting becomes a possibility...
We do not have a policy of doing this throughout the project, what we
have is a few places where we do it.
I propose that we should consider making a uniform policy decision for
the project - and specifically decide to use ungraceful reboots as our
recovery method for "key" processes dying (for example: CCM, heartbeat,
CIB, CRM). It should work for those cases where people don't configure
in watchdogs or explicitly define any STONITH devices, and also
independently of quorum policies - because AFAIK it seems like the right
choice, there's no technical reason not to do so.
My inclination is to think that this is a good approach to take for
problems that in our best-guess judgment "shouldn't happen".
I'm bringing this to both lists, so that we can hear comments both from
developers and users.
Comments please...
I would say the "right thing" would depend on your cluster
implementation and what is consider the right thing to do for the
applications that the cluster is monitoring.
I would propose that this action should be administrator configurable.
From a user point of view with the cluster that we are implementing we
would expect any cluster failure (internal) to either get itself back
and running or just send out an alert "Help me. im not working"... as we
would want our applications to continue running on the nodes. ** We dont
want a service outage just because the cluster is no longer monitoring
our applications. **
We would expect to get a 24x7 call out. Sev1 and then logon to the
cluster and see what was happening. (configured alerting)
Our applications only want a service outage if the node itself has
issues not the Cluster..
Here's the issue:
The solution as I see it is to do one of:
a) reboot the node and clear the problem with certainty
b) continue on and risk damaging your disks.
c) write some new code to recover from specific cases more
gracefully and then test it thoroughly.
d) Try and figure out how to propagate the failure to the
top layer of the cluster, and hope you get the notice
there soon enough so that it can "freeze" the cluster
before the code reacts to the apparent failure
and begins to try and recover from it.
In the current code, sometimes you'll get behavior (a) and sometimes
you'll get behavior (b) and sometimes you'll get behavior (c).
In the particular case described by bug 1762, failure to reboot the node
did indeed start the same resource twice. In a cluster where you have
shared disk (like yours for example), that would probably trash the
filesystem. Not a good plan unless you're tired of your current job
;-). I'd like to take most/all of the cases where you might get
behavior (b) and cause them to use behavior (a).
If writing correct code and testing it were free, then (c) would
obviously be the right choice.
Quite honestly, I don't know how to do (d) in a reliable way at all.
It's much more difficult than it sounds. Among other reasons, it relies
on the components you're telling to freeze things to work correctly.
Since resource freezes happen at the top level of the system, and the
top layers need all the layers under them to work correctly, getting
this right seems to be the kind of approach you could make into your
life's work - and still never get it right.
Case (c) has to be handled on a case by case basis, where you write and
test the code for a particular failure case. IMHO the only feasible
_general_ answer is (a).
There are an infinite number of things that can go wrong. So, having a
reliable and general strategy to deal with the WTF's of the world is a
good thing. Of course, for those cases where we have a (c) behavior
would not be affected by this change in general policy.
--
Alan Robertson <[EMAIL PROTECTED]>
"Openness is the foundation and preservative of friendship... Let me
claim from you at all times your undisguised opinions." - William
Wilberforce
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