The BtrPlace Constraints Catalog

MinSpareResources

Definition

Signature

minSpareResources(s : set<server>, rc : string, n : number)

s : a non-empty set of servers for a meaningful constraint.
rc : a resource identifier such as mem, ucpu, pcpu or nodes to identify the physical memory, the computational capacity, the physical CPUs or the node itself, respectively.
n : a positive number

The minSpareResources restricts to at least n, the number of free resources directly available for VMs on the online servers in s. Servers in the Offline state are ignored.

Classification

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

Usage

This constraint deserves the control of a power saving strategy. The most effective solution to reduce the energy consumption of a non-saturated datacenter is to turn off unused servers and to turn them on and off depending on the load variation. When a load spike arise, it may be a necessary to put some servers online to make their resources available to the VMs. [26], [48] The time to boot the awaited servers may however be significant and alter the reactivity of the datacenter when it faces an emergency situations. One solution is to let online a controlled number of spare resources that can be used instantly to absorb the load increase. A datacenter administrator may then use minSpareResources constraints to control the minimum number of free resources to let directly available while the other unused resources are still manageable by the power saving strategy.

Example

Figure 19 depicts a sample reconfiguration between a source and a destination configuration where each server provides 8 unit of CPU and 7 unit of memory resources to VMs. During the reconfiguration, several relocations have been performed and the server N3 has been turned off to save power. In this setting, the following minSpareResources constraints were considered:

figure 19: A reconfiguration motivated by minSpareResources constraints.

minSpareResouces({N1,N2,N3},"ucpu",0). This constraint was satisfied in the source configuration as 11 unit of CPU were directly available to the running VMs. The constraint is also satisfied in the destination configuration as 0 unit are available: resources on N3 are not considered as it is in the offline state.
minSpareResources({N2},"mem",1). This constraint was not satisfied in the source configuration as no memory resources were available on N2. The reconfiguration process fixed that violation by relocating away VM4 and VM3 and host VM2,VM1, and VM5 while keeping one unity of memory resources directly available.
minSpareResources({N2,N3,N4},"node",1). This constraint was not satisfied in the source configuration as no servers were idle. The reconfiguration fixed this violation by relocating all the VMs on N4 to other servers while keeping it in the Online state.