Aspects include selecting a channel group from a plurality of channel groups for an I/O operation. Each channel group in the plurality of channel groups is associated with a priority level and includes one or more channels. The selecting is based on a priority level assigned to the I/O operation and the priority level associated with the selected channel group. The I/O operation is driven on a selected channel in the selected channel group. A response time for the I/O operation is recorded and an average I/O response time for the selected channel is calculated. It is determined whether the plurality of channel groups should be reformed based at least in part on the calculated average response time for the selected channel. The plurality of channel groups are reformed in response to determining that the plurality of channel groups should be reformed.