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

Matthew Richardson m.richardson at ed.ac.uk
Mon Oct 4 09:01:50 MDT 2010


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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.

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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