Complete traffic failover between 2 priority groups

If all pool members in group A have a priority of 10, all for group B have a priority of 5, and you set priority group activation to less than 3:

As soon as there are less than 3 pool members available/enabled, all pool members of the next highest priority group will be added to the traffic distribution, regardless of how many of them there are.

In your scenario that means that the two remaining pool members for priority group 10 will be joined by all pool members with priority group 5.

If you want to start sending all new traffic to a different group of pool members you will need an iRule like:

when HTTP_REQUEST {
   if { [active_members pool_primary] < 3 } {
   pool pool_secondary
   }
}

where pool_primary includes the 5 pool members you had with priority 10 and pool_secondary includes the 5 pool members you had with priority 5.

If all pool members in group A have a priority of 10, all for group B have a priority of 5, and you set priority group activation to less than 3:

As soon as there are less than 3 pool members available/enabled, all pool members of the next highest priority group will be added to the traffic distribution, regardless of how many of them there are.

In your scenario that means that the two remaining pool members for priority group 10 will be joined by all pool members with priority group 5.

If you want to start sending all new traffic to a different group of pool members you will need an iRule like:

when HTTP_REQUEST {
   if { [active_members pool_primary] < 3 } {
   pool pool_secondary
   }
}

where pool_primary includes the 5 pool members you had with priority 10 and pool_secondary includes the 5 pool members you had with priority 5.