Re: [pve-devel] [PATCH ha-manager 05/15] rules: add colocation rule plugin

[Fiona Ebner <f.ebner@proxmox.com>]

Am 29.04.25 um 10:37 schrieb Daniel Kral:
> On 4/25/25 16:06, Fiona Ebner wrote:
>> Am 11.04.25 um 13:04 schrieb Daniel Kral:
>>> On 4/3/25 14:16, Fabian Grünbichler wrote:
>>>> On March 25, 2025 4:12 pm, Daniel Kral wrote:
>>> Also, I was curious about how that would work out for the case where a
>>> negative colocation rule was defined for three nodes with those rules
>>> split into three rules (essentially a cycle dependence). This should in
>>> theory have the same semantics as the above rule set:
>>>
>>> colocation: stick-together1
>>>      services vm:101,vm:104
>>>      affinity together
>>>      strict 1
>>>
>>> colocation: stick-together2
>>>      services vm:104,vm:102
>>>      affinity together
>>>      strict 1
>>>
>>> colocation: very-lonely-services1
>>>      services vm:101,vm:102
>>>      affinity separate
>>>      strict 1
>>>
>>> colocation: very-lonely-services2
>>>      services vm:102,vm:103
>>>      affinity separate
>>>      strict 1
>>>
>>> colocation: very-lonely-services3
>>>      services vm:101,vm:103
>>>      affinity separate
>>>      strict 1
>>>
>>> Without the merge of positive rules, 'check_inner_consistency' would
>>> again not detect the inconsistency here. But with the merge correctly
>>> applied before checking the consistency, this would be resolved and the
>>> effective rule set would be:
>>
>> I suppose the effective rule set would still also contain the two
>> 'together' rules, or?
> 
> No, here it would not. I found it would be most fair or reasonable that
> if a positive and a negative colocation rule contradict each other to
> drop both of them. Here the conflicts are
> 
> stick-together1 -- very-lonely-services1
> stick-together2 -- very-lonely-services1
> 
> so all three of them will be dropped from the rule set.
> 
> Seeing this again here, such cases definitely benefit from the immediate
> response with the 'conflict'/'ineffective' state to show users that
> those won't be applied instead of only logging it.

I don't think dropping all conflicting rules is best. Say you have a
rule between 100 services and that conflicts with a rule with just 2
services. Dropping the latter only is much preferred then IMHO. In
general, I'd argue that the more rules we can still honor, the better
from a user perspective. I don't think it's worth going out of our way
though and introduce much complexity to minimize it, because conflicts
are usually prevented while configuring already.

>>> colocation: very-lonely-services2
>>>      services vm:102,vm:103
>>>      affinity separate
>>>      strict 1
>>>
>>> colocation: very-lonely-services3
>>>      services vm:101,vm:103
>>>      affinity separate
>>>      strict 1
>>>
>>> It could be argued, that the negative colocation rules should be merged
>>> in a similar manner here, as there's now a "effective" difference in the
>>> semantics of the above rule sets, as the negative colocation rule
>>> between vm 101 and vm 103 and vm 102 and vm 103 remains.
>>>
>>> What do you think?
>>
>> I don't think there's a particular need to also merge negative rules
>> between services (when they form a complete graph). It won't make a
>> difference if there are no conflicts with positive rules and in edge
>> cases when there are conflicts (which usually gets caught while editing
>> the rules), it's better to drop fewer rules, so not merging is an
>> advantage. Or do you have a particular advantage in favor of merging in
>> mind?
> 
> Yes, I think so too.
> 
> There's quite the semantic difference between positive and negative
> colocation rules here. "Connected" positive colocation relationships
> (strict ones in particular) must be co-located in the end anyway, so it
> makes sense to merge them. Negative colocation relationships must be
> defined in a "circular" way and might just happen by coincidence for
> small scenarios.
> 
> But one thing that just struck me is that what if the user intentionally
> wrote them as separate rules? Then it might be confusing that all rules
> are dropped and not just the minimal amount that contradict other
> rules... Then check_inner_consistency() would just drop the minimal
> amount of rules that need to be dropped as in the above example.
> 
> It would be a softer interpretation of the rules indeed, but it might
> benefit the user in the end and make things easier to follow from the
> user perspective. If there's no opposition to that, I'd tend to drop the
> merging for any rules after all.

Having conflicts is already a bit of an edge case, so I don't think we
need to go out of our way to avoid merging of positive rules. But if it
doesn't increase the complexity much, it's fine either way IMHO.


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