Hi Duncan,

Really thanks for replying and sorry for my delayed response due to Chinese new 
year.

> 1) Driver authors tend, in my experience, to know more than admins, so
> drivers should be able (where useful) to be able to set a default value to
> either filter expression or weighting expression

Actually I am in a team which is writing cinder driver for Huawei storage 
array, and I agree that driver authors know more than admins about the storage 
device, but not the requirement. I think it's admin's choice what kind of 
storage is needed, e.g:
1) Admin A cares nothing but free capacity, so capacity filter is enough for 
him;
2) Admin B want SLA volume, so he need min_iops/min_bandwidth/replication to be 
filter conditions (in this case, the admin is likely to create a special 
volume-type);
I'm not sure if the scenario above is in the scope of this BP, but I think 
editing equation in cinder.conf IS a way to match the requirement.


> 2) Admins definitely need to be able to over-ride this if desired via
> cinder.conf

> I thing it is fairly easy (and beneficial) to go through the in-tree
> drivers and add the conf value to the stats report, once the base driver
> change has merged.

Do you mean the cinder base driver can have the built-in implementation of 
filter/goodness function? Is there a plan? maybe we can do something about this.
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