FailoverRatio: This field is applicable only when the containing target pool is serving a forwarding rule as the primary pool (i.e., not as a backup pool to some other target pool). The value of the field must be in [0, 1].
If set, backupPool must also be set. They together define the fallback behavior of the primary target pool: if the ratio of the healthy instances in the primary pool is at or below this number, traffic arriving at the load-balanced IP will be directed to the backup pool.
In case where failoverRatio is not set or all the instances in the backup pool are unhealthy, the traffic will be directed back to the primary pool in the "force" mode, where traffic will be spread to the healthy instances with the best effort, or to all instances when no instance is healthy.