[Linux-HA] Documentation for constraints

[Ragnar Kjørstad]

I'm trying to understand the finer details of the constraints, but there
are some wholes in the documentation:

I would like to suggest the following changes:

http://wiki.linux-ha.org/v2/dtd1.0/annotated#head-390c0a5ecce666978dab397e20d1575ff366c262

should include more exact information about what implicit rules the
group implements.

Is it correct that a group with resources x and y imply rules:
<rsc_colocation id="foo" from="y" to="x" score="INFINITY"/>  
<rsc_order id="bar" from="x" action="start" type="before" to="y" symmetrical="TRUE"/>



http://wiki.linux-ha.org/v2/dtd1.0/annotated#head-390c0a5ecce666978dab397e20d1575ff366c262

needs to explain type better. E.g "x start before y" is the same as "y
start after x"?

The explanation for symmetrical is also not good enough, because it's
not clear what the "reverse" constraint is.

Explaining that 
<rsc_order id="foo" from="x" action="start" type="before" to="y" symmetrical="TRUE"/>
is identical to the following two rules:
<rsc_order id="foo" from="x" action="start" type="before" to="y" />
<rsc_order id="foo" from="x" action="stop" type="after" to="y" />

would go a long way to explain this.



http://wiki.linux-ha.org/v2/dtd1.0/annotated#head-390c0a5ecce666978dab397e20d1575ff366c262

needs to explain if "x colocated with y" means the same thing as "y
colocated with x" or not. Is it correct that 
<rsc_colocation id="foo" from="x" to="y" score="INFINITY"/> 
affects the score of x, but not y.

Does that mean that y will be placed first, and x will follow?
What if y is placed on a node where x fails to start, will both x and y
be migrated to another node?

What is the best practise use of colocation? should you normally
colocate resources with the resources they depend on (e.g. application
with filesystem)? 

What about colocation rules with -INFINITY score - are they assymetrical
as well?


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