Abstract: To manage a storage system for storing write data of I/O (Input/Output) command to a storage volume, a computer program comprises: code for analyzing performance information of I/O operation for a period of time on a storage volume basis; code for deriving a periodic time window having a same type of I/O performance characteristic; code for determining a type of Service Level Objectives (SLO) on a periodic time window basis; code for calculating a threshold value of the SLO; code for providing a user with a type of SLO for a periodic monitoring window and a threshold value of SLO for the periodic monitoring window on a storage volume group basis; and code for monitoring, on a storage volume basis, whether or not a service level value for the periodic monitoring window violates the SLO based on the threshold value of SLO for the periodic monitoring window.