| ================================== |
| Migration speed accounting in Hbal |
| ================================== |
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| .. contents:: :depth: 2 |
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| Hbal usually performs complex sequence of moves during cluster balancing in |
| order to achieve local optimal cluster state. Unfortunately, each move may take |
| significant amount of time. Thus, during the sequence of moves the situation on |
| cluster may change (e.g., because of adding new instance or because of instance |
| or node parameters change) and desired moves can become unprofitable. |
| |
| Usually disk moves become a bottleneck and require sufficient amount of time. |
| :ref:`Instance move improvements <move-performance>` considers |
| disk moves speed in more details. Currently, ``hbal`` has a ``--no-disk-moves`` |
| option preventing disk moves during cluster balancing in order to perform fast |
| (but of course non optimal) balancing. It may be useful, but ideally we need to |
| find a balance between optimal configuration and time to reach this |
| configuration. |
| |
| Avoiding insignificant disk moves |
| ================================= |
| |
| Allowing only profitable enough disk moves may become a first step to reach |
| a compromise between moves speed and optimal scoring. This can be implemented |
| by introducing ``--avoid-disk-moves *FACTOR*`` option which will admit disk |
| moves only if the gain in the cluster metrics is *FACTOR* times |
| higher than the gain achievable by non disk moves. |
