Taking relative CPU speed into account







This document describes the suggested addition of a new node-parameter, describing the CPU speed of a node, relative to that of a normal node in the node group.

Current state and shortcomings

Currently, for balancing a cluster, for most resources (disk, memory), the ratio between the amount used and the amount available is taken as a measure of load for that resources. As hbal tries to even out the load in terms of these measures, larger nodes get a larger share of the instances, even for a cluster not running at full capacity.

For for one resources, however, hardware differences are not taken into account: CPU speed. For CPU, the load is measured by the ratio of used virtual to physical CPUs on the node. Balancing this measure implicitly assumes equal speed of all CPUs.

Proposed changes

It is proposed to add a new node parameter, cpu_speed, that is a floating-point number, with default value 1.0. It can be modified in the same ways, as all other node parameters.

The cluster metric used by htools will be changed to use the ratio of virtual to physical cpus weighted by speed, rather than the plain virtual-to-physical ratio. So, when balancing, nodes will be considered as if they had physical cpus equal to cpu_speed times the actual number.

Finally, it should be noted that for IO load, in non-dedicated Ganeti, the spindle_count already serves the same purpose as the newly proposed cpu_speed. It is a parameter to measure the amount of IO a node can handle in arbitrary units.