The BtrPlace Constraints Catalog

MaxOnlines

Definition

Signature

maxOnlines(s : set<server>, n : number)

s : a non-empty set of servers for a meaningful constraint.
n : a positive number, inferior to the number of servers in s.

The maxOnlines ensures the number of online servers in s is inferior or equals to n.

Classification

Primary users: datacenter administrator
Manipulated elements: Resource allocation
Concerns: Resource management, Power saving, Capacity planning

Usage

This constraint deserves the control of the datacenter hosting capacity. A datacenter may be composed of servers that differ in their hardware capacity or performance. It may however not be possible to keep all the servers online simultaneously. As an example, the cooling system or the power distribution unit may restrict the number of online servers due to its delivering capacity. Licenses restrictions, such as the per-server license model of XenServer, may also limits the number of nodes that are online simultaneously. [1]

In this setting, once the maximum number of online servers reached, turning on one additional server to use its specificities is only allowed if an online server can be turned off in exchange. In this setting, a datacenter administrator may then use of maxOnlines constraints to control the number of online servers and automatically manage their state if needed.

Example

Figure 21 depicts a sample reconfiguration between a source and a destination configuration where only servers N1, N2 and N3 are online in the source configuration. Each VM is associated to a gray rectangle that denotes its resource usage in terms of memory and UCPU. In the source configuration, the server N1 was saturated has VM4 and VM6 were competing for the same resources. The reconfiguration process fixed this violation but also consider the following maxOnlines constraints:

figure 21: A reconfiguration motivated by maxOnlines constraints.

maxOnlines({N1,N2, N3, N4}, 3). This constraint was satisfied in the source configuration as three over the four servers were in the Online state. The constraint is still satisfied in the destination configuration. N4 has been turned on to host VM6 but N3 has been turned off in exchange according to the constraint specification. To be able to turned off N3, VM1 has been relocated to N2. Figure 3.22 depicts the associated event-based reconfiguration plan.

Ø relocate(VM1)
!relocate(VM1) halt(N3)
!halt(N3) boot(N4)
!boot(N4) relocate(VM6)
Figure 3.22: Event-based reconfiguration plan associated to the reconfiguration depicted in Figure 21.
maxOnlines({N1, N3},1). This constraint was not satisfied in the source configuration as the two servers were in the Online state. The reconfiguration process fixed this violation by turning off N3.