Priority Group Activation fail secondary log

if the one of group10 unavailable,it fails group 5,i understand 2 or more pool members in group 10 have to be down before group 5 will be used.

 

 

if group 5 be use, How can I use iRule to log?there is LB::server priority command which returns priority group of selected pool member. could it be useful?

 

 

LB::server

 

https://devcentral.f5.com/wiki/irules.LB__server.ashx