The BtrPlace Constraints Catalog

Spread

Definition

Signature

spread(s : set<VM>)

s : a set of at least 2 VMs for a meaningful constraint. VMs not in the Running state are ignored.

The spread constraint forces all the running VMs in s to be hosted on distinct servers at any time, even during the reconfiguration process.

Classification

Primary users: application administrator
Manipulated elements: VM placement, Actions schedule
Concerns: Fault tolerance, VM-to-VM placement, Partitioning

Usage

The spread constraint may be used by an application administrator to provide to a replicated service, fault tolerance to hardware failures. By hosting each replicas on a distinct server, the service will be available while at least one server is still online. To achieve this purpose, one spread constraint can be used with the replicas provided as arguments.

Example

Figure 10 depicts a sample reconfiguration between a source and a destination configuration. In this example, the following spread constraints were considered:

N1: VM1 VM2

N2: VM3 VM4 VM5

N3: VM6

N1: VM1 VM6

N2: VM3 (VM4)

N3: VM2 VM5

Figure 10: A reconfiguration motivated by spread constraints.

spread({VM1,VM2}). This constraint was not satisfied in the source configuration as both VMs were colocated. The reconfiguration fixed this violation by relocating VM2 to N3.
spread({VM3, VM4}). This constraint was not satisfied in the source configuration. Putting VM4 in the Suspended state fixed this violation without having to perform a relocation.
spread({VM5, VM6}). This constraint was satisfied in the source configuration. However, let consider VM5 must be running on N3, which was already hosting VM6. In this setting, VM6 has been relocated to N1 to disallow the colocation. Furthermore, to prevent from a temporary colocation on N3, it was a necessary to relocate VM6 before VM5. Figure 3.11 depicts the associated event-based reconfiguration plan.

Ø	stop(VM4) & relocate(VM5)
!relocate(VM5)	relocate(VM6)
!relocate(VM6)	relocate(VM2)

Figure 3.11: Event-based reconfiguration plan associated to the reconfiguration depicted in Figure 10.