1.Can cloudstack allocate vm instances averagely between host nodes?

The default value of vm.allocation.algorithm is "random".
If there is just one host node in a cluster(we call it A) and I create 20 
instances,all of them will be allocated on host A.Now I add another host B,and 
create 30 instance again,what will happen?
I think the number of instances on A is 35,on B is 15,according to probability 
theory.But I hope the result is 25 on A,25 on B,but I don't know how to 
configure cloudstack to achieve it.

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2.The similar question about volume allocating.

If I have two empty primary storage pools in a cluster,the volumes of vms will 
be allocated randomly on both of them.
But If I have a primary storage with 20 volumes and an empty primary 
storage,how can I let new volumes allocated 
averagely on them?

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3.How to set a standby host in a cluster?
There is there host nodes A,B,C ,vm instances are allocated randomly on them.
Now I want to add a standby host node D into the cluster.
If there are enough compute resources on A,B,C ,no new instance will be 
allocated on D.
But if A breaks down and there are not enough resources on B,C , many of the 
vms on A will be migrated to D.
Is it possible to set a standby host like this?

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