How to properly set a root disk for the overcloud nodes based on disk size ?

Issue

Overcloud nodes have multiple disks attached to them, how to set the root disk for overcloud nodes based on the size of the disk ?
There are 2 disks attached to the overcloud nodes, one disk is of size 41 GiB and the other one is of size 20 GiB, how to set the disk of size 41 GiB as the root disk ?

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