MostlySpread

Definition

Signature

mostlySpread(s : set<VM>, n : number)

• s : a non-empty set of virtual machines for a meaningful constraint

• n: a positive number, inferior to the number of virtual machines in s

The mostlySpread constraint ensures the running virtual machines in s will be running on at least n distinct servers.

Classification

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

Usage

The mostlySpread constraint may be used by an application administrator to provide to a replicated service, fault tolerance to hardware failures. By hosting replaces on distinct servers, the service will be available while at least one server is still online. When the number of replicas is important, it is however difficult to have a large amount of different servers. Furthermore, the chances of having all the hosting hervers but one failing simultaneously decrease when the number of replicas increases. It is then tolerable to use a number of servers that is smaller to the number of replicas. The application administrator can then use one mostlySpread constraint to indicate the minimum number of distinct servers that must be used to host the replicas.

Example

Figure 13 depicts a sample reconfiguration between a source and a destination configuration. In this example, the following mostlySpread constraints were considered:

N1: VM1 VM2

N2: VM5

N3: VM3

N1: VM1 VM6

N2: VM2

N3: VM3 VM5

Figure 13: A reconfiguration motivated by mostlySpread constraints.

• mostlySpread({VM1, VM2, VM5}, 2). This constraint was satisfied in the source configuration as all the VM were running on 2 distinct servers. The constraint is still satisfied in the destination configuration as all the VMs are running on 3 distinct servers.

• mostlySpread({VM1,VM2}, 1). This constraint was not satisfied in the source configuration as the VMs where running on N1. The relocation of VM2 to N3 fixed this violation.

See also

Related Constraints

• spread: a constraint that guarantees the VMs will never overlap on a same server, even during the reconfiguration process.

• lazySpread: a constraint similar to mostlySpread but that guarantee every VMs are running on distinct servers.

Specialization(s)

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