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

[Michael Schwartzkopff]

Am 20.11.18 um 08:57 schrieb 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?
>
Everytime. That is what causes the load.



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