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Historical performance data
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Use case
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OpenStack is really rapidly developed. Hundreds of patches are merged daily
and it's really hard to track how performance is changed during time.
It will be nice to have a way to track performance of major functionality
of OpenStack running periodically rally task and building graphs that represent
how performance of specific method is changed during the time.


Problem description
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There is no way to bind tasks


Possible solution
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* Add grouping for tasks
* Add command that creates historical graphs
