%b - Percentage of time the disk is busy (transactions in progress). Average %b values over 25 could be a bottleneck.
%w - Percentage of time transactions are waiting for service (queue non-empty).
asvc_t - Reports on average response time of active transactions, in milliseconds. This option is mislabeled asvc_t; it indicates the time between a user process issuing a read and the read completing. Consistent values over 30ms could indicate a bottleneck.