[ClusterLabs] Antw: Re: Antw: Placing resource based on least load on a node

[Ulrich Windl]

>>> Michael Schwartzkopff <ms at sys4.de> schrieb am 20.11.2018 um 08:41 in Nachricht
<e37932d6-add8-63ce-14c7-dd71df12e203 at sys4.de>:
> Am 20.11.18 um 08:35 schrieb Bernd:
>> Am 2018-11-20 08:06, schrieb Ulrich Windl:
>>>>>> Bernd <bernd at kroenchenstadt.de> schrieb am 20.11.2018 um 07:21 in
>>>>>> Nachricht
>>> <dbae607c63168d4e14584abfba0b4b1b at kroenchenstadt.de>:
>>>> Hi,
>>>>
>>>> I'd like to run a certain bunch of cronjobs from time to time on the
>>>> cluster node (four node cluster) that has the lowest load of all four
>>>> nodes.
>>>>
>>>> The parameters wanted for this system yet to build are
>>>>
>>>> * automatic placement on one of the four nodes (i.e., that with the
>>>> lowest load)
>>>>
>>>> * in case a node fails, automatically removed from the cluster
>>>>
>>>> * it must only exist a single entity of the cronjob entity running
>>>>
>>>> so this really screams for pacemakter being used as foundation.
>>>>
>>>> However, I'm not sure how to implement the "put onto node with least
>>>> load" part. I was thinking to use Node Attributes for that, but I
>>>> didn't
>>>> find any solution "out of the box" for this. Furthermore, as load is a
>>>> highly volatile value, how can one make sure that all cronjobs are run
>>>> to the end without being moved to a node that possibly meanwhile got a
>>>> lower load than the one executing the jobs?
>>>
>>> Hi!
>>>
>>> Actually I think the last one is the easiest (assuming the cron jobs
>>> do not need any resources that are moved): Once a cron job is started,
>>> it will run until it ends, whether it's crontab has been moved or not.
>>>
>>> Despite of that I think cluster software is not ideal when you
>>> actually need load-balancing software.
>>>
>>> Regards,
>>> Ulrich
>>
>> The only resource(s) existing would be the cron "runner".
>>
>> The point about load balancing is true, yes... so, any idea what to
>> use instead? Is there already a tool or framework for solving a
>> problem like this available or do I have to start from scratch? Not
>> that I'd be too lazy, but what's the use of reinventing the wheel
>> repeatedly...? ;)
>>
>> Regards,
>>
>> Bernd
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> 
> 
> hi,
> 
> 
> I solved this problem years ago. I used the utilization attribute. But
> you can use any attribute. You have to write an agent that measures the
> CPU load every X minutes and updates the attribute.  Now you just have
> to add a location constraint, that starts the resource on the node with
> the "best" attribute value. The "best" could be lowest CPU usage or most
> free RAM or whatever you want.
> 
> 
> The disadvantage of this solution is that the cluster (i.e. pacemaker)
> has to recalculate the scores every time you update your attribute. That
> causes additional load. If you have many resources the interdepend that
> additional load may be not negligible.

Hi!

Question on this: Is the cluster clever to check only updates of attributes that some rule actually uses, or does it re-evaluate everything when any attribute changed?

Regards,
Ulrich