how to get 'peer disk' in raid configuration?

[George Shuklin <george.shuklin@xxxxxxxxx>]

Few days ago met intersting question:

Let assume we have RAID10 on 4 disks. We want to remove two  disks 
without  breaking array.

How we can know which drive is safe to eject? For 1st drive it obvious: 
any of. Second is more complicated: we need eject mirror part of 'second 
pair'.

How we can know exact topology of raid? (--detail does not provide 
enough information, IMHO).
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