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From Hari Kannan <hari.kan...@citrix.com>
Subject RE: [Discuss] Cpu and Ram overcommit.
Date Wed, 26 Dec 2012 21:06:11 GMT
What should the behavior be if admin changes the overcommit factor for a cluster that conflicts
with the current situation. For example, lets assume Cluster X has an over commit factor of
1.5x for memory and the admin wants to change this to 1x - i.e no overcommit (or changes from
2x to 1.5x) - however, based on the "older" factor, CS might already have assigned more VMs
- when the admin reduces the overcommit value

1. if there is no conflict, there is no issue
2a. if there is a conflict (i.e. current allocation would conflict with the new value) - should
we reject this change?  (preferred)
2b. or accept the change but not add more VMs anymore

-----Original Message-----
From: Bharat Kumar [mailto:bharat.kumar@citrix.com] 
Sent: Wednesday, December 26, 2012 4:39 AM
To: cloudstack-dev@incubator.apache.org; cloudstack-users@incubator.apache.org
Subject: Re: [Discuss] Cpu and Ram overcommit. 

Nitin thanks for your suggestions.

My comments inline

On Dec 26, 2012, at 3:22 PM, Nitin Mehta <Nitin.Mehta@citrix.com> wrote:

> Thanks Bharat for the bringing this up. 
> I have a few questions and suggestions for you.
> 
> 1. Why do we need it per cluster basis and when and where do you configure this ? I hope
when we change it for a cluster it would not require MS reboot and be dynamically understood
- is that the case ?
    Depending on the applications running in a given cluster the admin needs to adjust the
over commit factor. for example if the        applications running in a cluster are ram intensive
he may want to decrease the ram overcommit ratio for this cluster without effecting the other
clusters. This can be done only if the ratios can be specified on a per cluster basis. 
Also to change these ratios MS restart will not be required.
 
> If we make it cluster based allocators will have to check this config for each cluster
while allocating and can potentially make allocators expensive. Same logic applies for dashboard
calculation as well.
> What granularity and fine tuning do we require - do you have any use cases ?
   The intent of having cluster based over provisioning  ratios is to deploy VMs selectively
depending on the type of application the vm will run. By selectively i mean the admin will
want to specify in which clusters to run the VM. This will narrow down the number of clusters
we need to check while deploying.  I still don't know the exact way in which we should control
the vm deployment. This definitely needs further discussion, will be clear once we narrow
down all the possible use cases.


> 2. What would happen in case of contention ?
In case of contention the the hypervisor specific methods to handle the contention will come
into effect. This feature assumes that admin has thought of the possible scenarios and has
chosen the overcommit ratios accordingly.
> 
> 3. Please remember to take care of alerts and dashboard related functionality. Along
with this also list Zone/Pod.../host/pool API also use this factor. Please make sure that
you take care of that as well.
Thanks for the suggestions. 

> 
> -Nitin
> 
> On 26-Dec-2012, at 11:32 AM, Bharat Kumar wrote:
> 
>> Hi all,
>> 
>> Presently in Cloudstack  there is a provision for cpu overcommit and no provision
for the ram overcommit. There is no way to configure the overcommit ratios on a per cluster
basis.
>> 
>> So we propose to add a new feature to allow the ram overcommit and to specify the
overcommit ratios ( cpu/ram ) on a per cluster basis. 
>> 
>> Motivation to add the feature:
>> Most of the operating systems and applications do not use the allocated resources
to 100%. This makes it possible to allocate more resource than what is actually available.
 The overcommitting of resources allows to run the  underutilized VMs in fewer number of hosts,
This saves money and power. Currently the cpu overcommit  ratio is a global parameter which
means there is no way to fine tune or have a granular control over the overcommit ratios.

>> 
>> This feature will enable 
>> 1.) Configuring the overcommit ratios on a per cluster basis.
>> 2.) ram overcommit feature in xen and kvm. ( It is there for VMware.) 
>> 3.) Updating the overcommit ratios of a cluster.
>> 
>> Regards,
>> Bharat Kumar.
> 


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