[Pacemaker] Very strange behavior on asymmetric cluster

Pavel Levshin pavel at levshin.spb.ru
Mon Mar 21 13:52:16 EDT 2011

21.03.2011 20:14, Carlos G Mendioroz:
>> It is used dynamically quite often. For example moving resource out of
>> one node creates a such location rule. Does it mean that along with
>> moving resource Pacemaker has to stop monitoring it on the left node?
>> I don't think so.

You are right, location rules is not suitable for this case. I'd prefer 
an additional meta parameter (or two) for the resource, listing 
"included" or "excluded" nodes.

> Neither do I. That was exactly my precondition :)
> Being that the RA absence is dealt with ok (i.e. no need to
> install the RA to enable pacemaker to do what it needs) then I feel
> it's ok anyway.

It's not completely OK.

First, personally I have been in sutuation when rc=5 "not installed" had 
been lost due to (still existing) bug 
(http://developerbugs.linux-foundation.org/show_bug.cgi?id=2568). It is 
a particular case which will eventually be fixed. But there are other 
possibilities to get into similar situation. Why wait for disaster?

Second, RA is not resource. You may have two independent resources with 
one RA, suitable for different nodes. You can overcome this by copying 
the RA and accessing it by different names for each resource. It would 
lead to the case #1.

Third, deleted RA may resurrect after software upgrade. You can defend 
yourself against this by using nonstandard location for your RAs. It may 
be considered good practice anyway, but IMHO this 'best practice' is not 
described in documentation.

All of this makes building highly available cluster more difficult.

> I've seen many times arguments of the kind "if the admin does this,
> then it breaks". I buy no such argument. I'm against systems playing
> smarter than admins.

So am I. Currently, the system tries to auto-detect resource existence 
by probing it, when admin knows that resource cannot exist there.

Pavel Levshin

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