There is no concept of dedicated spare in Virtual Array. So whether you configure Global spare or Dynamic Spare feature both will be at the system level.
So if your system having same type different size drives then always there is a chance of space wastage when smaller size drive failed and higher size drive configured as spare.
That is why we always suggest to have same type and same size drives to get optimum performance.
Now coming to balancing to load accross the pools that depends up on your business requirement and applications.
Hope this helps!
Regards
Subhajit
I am an HPE employee
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