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• VM's current host has capacity - If VM's current host has capacity to scale up the vm then we put the vm in Reconfiguring state and lock the delta capacity. We then send the ReconfigureVMCommand to the HV to reconfigure the vm and scale it to the new values. Whether success / failure we put the vm back into running state, but release the delta capacity in case of failure.
• VM's current host doesn't have capacity but the vm's cluster has - If VM's current host doesnt have capacity, then we call up the planners to find a suitable host that can take the scaled up vm in the cluster. Once the host is found out we lock the new required capacity on the new host and migrate the vm to that host. Once migrated we send ReconfigureVMCommand to the HV. IF there is failure in reconfiguring here then we release the delta capacity on the new host.
• Cluster in which vm is running doesn't have capacity - we simply return failure to the end user saying that we dont have enough capacity to scale up the vm.

For dynamic scaling to work virtual machine should have XS tools / VMware tools installed on it. To ensure this Admin/User should provide an input whether tools are installed on the template(during registering the template or can be done using update template API) that is used to deploy vm. If the user deploys a virtual machine with a template that does not have XS tools / VMware tools and later if tools are installed on the VM then this can informed to Cloudstack by updating the virtual machine (using updatevirtualmachine API). After installation of tools and updating the virtual machine, user needs to stop and start the vm from cloudstack in order to work dynamic scaling of CPU and RAM for that VM. The reason why we need to stop start the VM after updating is we need to set static max memory to some higher value before VM starts so that we can dynamically scale the VM upto static max.

Web Services APIs

Following APIs needs to be changed:

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updateVirtualMachine - An additional optional boolean parameter "isdynamicallyscalable" is introduced, the value is true if VM contains XS/VMWare tools inorder to support dynamic scaling of VM cpu/memory

Xenserver /Vmware Tools

For dynamic scaling to work virtual machine should have XS tools / VMware tools installed on it. To ensure this Admin/User can do it in two ways :-

• Admin/User while registering the template provides an input whether tools are installed on the template(or can be done using update template API).
• If the user deploys a virtual machine with a template that does not have XS tools / VMware tools and later if he/she installs the tools on the VM then he can inform Cloudstack using using updatevirtualmachine API. After installation of tools and updating the virtual machine, user needs to stop and start the vm from cloudstack in order for dynamic scaling of CPU and RAM for that VM. The reason why we need to stop start the VM after updating is we need to set static max memory to some higher value before VM starts so that we can dynamically scale the VM upto static max.

Hypervisor Changes

Xenserver

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• new dynamic max (y) <= static max and  
• new dynamic min (y / memory_overprovisioning_of_cluster) >= static min.

Vmware -
DeployVmCmd - 

During the initial deployment of the VM we enable the flags hotAddCpu and hotAdd memory true. These flags will be turned on if the guest OS supports it.

ScaleVmCmd - 

During scaling up of vm, the new requested RAM and CPU are set if the flags are turned on.

DB Changes

• Introducing a global config - enable.dynamic.scale.vm. Enables/Diables dynamically scaling a vm. This can be used to turn off the feature and is available at the zone level. By default the feature is turned off for upgrade reasons.
• Introducing a global config - scale.retry. By default the value = 2. This is for number of tries before failing the scaling.
• A new column in "vm_template" table named "dynamically_scalable" of type tinyint(1) and the value is 1 if template contains XS/VMWare tools inorder to support dynamic scaling of VM cpu/memory otherwise 0
• "user_vm_details" table holds key value pair with key "enable.dynamic.scaling" and value is true if VM contains XS/VMWare tools inorder to support dynamic scaling of VM cpu/memory otherwise false

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• Created a subtask for this https://issues.apache.org/jira/browse/CLOUDSTACK-1041
• During template registration/updation there should be a check box to know whether tools are installed on the template or not.  Same check box should be there for updateVirtualMachine API. https://issues.apache.org/jira/browse/CLOUDSTACK-2988^{Image Removed}

Open Issues

• Should scale down be allowed? It can be explicitly prevented since none of the HVs/guest OS supports it.

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