| ====================================== |
| Taking relative CPU speed into account |
| ====================================== |
| |
| .. contents:: :depth: 4 |
| |
| 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 implictly 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. |
