LazySpread

Definition

Signature

lazySpread(s : set<VM>)

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

The lazySpread constraint forces all the running VMs in s to be hosted on distinct servers at the end of a reconfiguration process.

Classification

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

Usage

The lazySpread 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 lazySpread constraint can be used with the replicas provided as arguments.

Example

Figure 12 depicts a sample reconfiguration between a source and a destination configuration. In this example, the following lazySpread constraints were considered:

N1: VM1 VM2

N2: VM4

N3: VM3

N1: VM1

N2: (VM4) VM3

N3: VM2

Figure 12: A reconfiguration motivated by lazySpread constraints.

• lazySpread({VM1, VM3, VM4}). This constraint was satisfied in the source configuration as each VM were running on distinct servers. The constraint is still satisfied in the destination configuration despite VM3 and VM4 are colocated as VM4 has been suspended.

• lazySpread({VM1,VM2}). This constraint was not satisfied in the source configuration as both VMs where running on N1. The relocation of VM2 to N3 fixed this violation.

• lazySpread({VM2, VM3}). This constraint was satisfied in the source configuration as both VMs were running on distinct servers. The constraint is still satisfied in the destination configuration despite the relocation of VM2 to N3 that was running VM3 initially as VM3 has been relocated elsewhere to let both VMs be running on distinct servers at the end of the reconfiguration process.

See also

Related Constraints

• gather: the opposite constraint of spread

• spread: a constraint similar to lazySpread but which also guarantee that the VMs will never overlap on a same server, even during the reconfiguration process.

Reformulation(s)

• Using mostlySpread: lazySpread(s) ↔ mostlySpread(s,|s|)

• Using splitAmong: lazySpread(s1) ↔ splitAmong(s/|s|,N/|N|)

• Using split: lazySpread(s) ↔ split(s/|s|)