[Linux-HA] Handling colocation constraints with more than 2 entries

Dejan Muhamedagic dejanmm at fastmail.fm
Mon Oct 4 09:36:17 MDT 2010


Hi,

On Mon, Oct 04, 2010 at 04:01:50PM +0100, Matthew Richardson wrote:
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> I've been playing with pacemaker for a while now, and have recently
> seena  user stung by an issue I had when I first started - namely that
> colocation constraints are limited to 2 entries, unless sets are used.
> 
> Thus:
> 
> colocation ok inf: A B
> 
> is allowed (obviously!)
> 
> colocation sets_ok inf: A (B) (C)
> 
> is also allowed.
> 
> However:
> 
> colocation not_ok inf: A B C
> 
> isn't valid, though a user might expect it to be equal to the set-based
> contraint.

Hmm, last time I looked it worked. How do you know that it's not
valid?

Thanks,

Dejan

> I would like to suggest 2 potential solutions to this:
> 
> 1) (simple) Provide a warning/error message when someone constructs this
> invalid constraint.
> 
> 2) (more complex) Translate this constraint to a meaningful set - i.e
> change 'A B C' to 'A (B) (C)'
> 
> I'm not sure whether or not the 2nd option makes sense or whether it
> adds some extra level of confusion or uncertainty to its behaviour.
> 
> Any comments?  I'm happy to do some work to submit a patch to the shell
> to at least do the basic checking, though this might not be the best
> place (or indeed the best patch) to achieve these, if people think
> they're worthwhile suggestions.
> 
> Thanks,
> 
> Matthew
> 
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