Root

Definition

Signature

root(s : set<VM>)

• s : an non-empty set of VMs for a meaningful constraint. VMs not in the Running state are ignored.

The root constraint forces each running VM in s to not move from its current location. This constraint only restricts the placement of running VMs with regards to a previous placement. As a result, it is not possible to state for the satisfaction of one root constraint before a reconfiguration occurred.

Classification

• Primary users: application administrator, datacenter administrator
• Manipulated elements: VM placement
• Concerns: VM-to-server placement, Performance, Resource management

Usage

The root constraint is mostly used to disallow the relocation of VMs when it is not possible or tolerated. Typically, a running VM may be attached to a peculiar device such as a filesystem or a PCI device on its host. In this setting, the relocation of the VM may not maintain this link and should not be performed for reliability reasons. An application administrator may then use a root constraint on this constraint to prevent from relocation. In addition, an application administrator may disallow the relocation of his VMs to prevent from the temporary performance loss that occur during this action.

Another possible usage is to disallow VMs relocation when the infrastructure or the underlying hypervisor does not support it. In this setting, the datacenter administrator may use a root constraint on all the VMs to disallow their relocation.

Example

Figure 1 depicts a sample reconfiguration between a source and a destination configuration. In this example, the following root constraints were considered:

N1: VM1 VM2

N2: VM3 VM5

?: VM4

N1: VM1 VM4

N2: VM3 VM2

?: VM5

Figure 1: A reconfiguration motivated by root constraints.

• root({VM1,VM3}). This constraint is satisfied as none of the VMs were relocated during the reconfiguration

• root({VM4, VM5}). This constraint is satisfied as VM4 is not running in the initial configuration while VM5 is no longer running in the destination configuration. The constraint ignores then these VMs.

• root({VM2}). This constraint is not satisfied as VM2 has been relocated from N1 to N2 during the reconfiguration process.

See also

Related Constraints

• fence: The root constraint can be emulated using a fence constraint when its user knows the current host of the specified VMs. For each VM, the list of servers given in the fence constraint is a singleton only composed of the current hosting server.