[Linux-HA] Documentation for constraints

[Ragnar Kjørstad]

On Thu, Mar 22, 2007 at 07:40:31PM +0100, Dejan Muhamedagic wrote:
> > The unclear part is that there is 3 things that can be reversed.
> > "x" vs "y", "start" vs "stop" or "before" vs "after".
> 
> the first and the last don't make sense.

Ok, so it wasn't _that_ confusing. But I don't think the tiny
clarification hurts. If you feel differently, feel free to remove it.

> > Interesting. Any more information?
> > The changelog includes
> >   + Support weak and uni-directional collocation constraints (FATE
> >     300792).
> > but I see no mention of "weak" or "uni-directional" in the DTD or
> > anywhere else in the documentation.
> 
> unfortunately our master of crm is not available right now. i'll
> try to explain what it is about: a group of resources or an
> ordered set of resources used to behave like a unit. one of them
> goes down, the others follow. that hasn't been quite right,
> because one would expect those resources which are "under" (in
> terms of order, started before) to keep running. in other words, a
> colocation should mean that the two things, if they run, must run
> on the same node, but it should not imply that they depend on each
> other. that was particularly important for non-grouped resources,
> because one common scenario is that two resources which are
> independent of each other both depend on a third one.

This is _exactly_ what I'm taking about. 

I think this issue applies to groups as much as non-grouped resources,
but I expect we will have to use explist constraints instead of groups
anyway to be able to do things like two different application both
depending on a database but the two applications are equal and we don't
really want to put one before another in a group.

> i'm afraid that the documentation hasn't been updated.

So, anyone know how to use this new weak and uni-directional colocation
feature?

> > > > What is the best practise use of colocation?
> > > 
> > > not sure if i understand this. basically, you just use them where
> > > you need them. hmm, that sounds stupid, but don't see any other
> > > way to say it :)
> > 
> > I should probably be more explisit:
> > In an example with resource A(filesystem), B(database), C(webserver)
> > where A must be started before B before C on the same host. What would
> > the natural colocation rules be?
> 
> you're talking about order here. one can only assume that ABC
> should run on the same host.

Nah, I am talking about both ordering and colocation (that's what I
meant by "same host").

> > If the colocation rules are all bi-directional and it doesn't matter
> > what order it is specified in it's pretty simple, but belive Alan
> > explained it wasn't that simple.
> 
> alan?

So maybe Alan was refering to the new weak uni-directional colocation
rules? Makes sense, since it appears to be what we need. Alan? :-)


-- 
Ragnar Kjørstad
Software Engineer
Scali - http://www.scali.com
Scaling the Linux Datacenter