If I have understood your Q correctly, depending on your Z system 'model', apart from a few older 'BC' systems, all Z Systems have dedicated 'spare' PU/PUs (processing units). If any CP, IFL, ICF, zIIP or SAP fails, 'sparing' action will take place transparently and in your example the 'SPARE' PU will characterized as a CP. How quickly does this happen? You won't even notice it. There is a lot more to this. If for some reason there aren't any 'SPARE PUs' left, then one of the uncharactized PUs i.e. not purchase by the customer will be used by the 'sparing' action. In case of multiple 'PU' failures (CP, ICF, zIIP etc) , there is a pecking order as to which one will be 'recovered' first. This is documented in the Redbooks I co-authored. Sparing is transparent to the operating system and applications.
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