A computer system is provided with a file system storing data; a plurality of I/O nodes which are adapted to access the file system; a compute node adapted to execute a job and to issue an I/O request when requiring an I/O operation; and a job server for job scheduling which dynamically allocates an I/O resource of the I/O nodes to a job without stopping execution of the job. The job server includes an I/O node scheduler adapted to, when being not able to fully secure an desired amount of the I/O resource of the I/O nodes required by the job in starting the job, secure a part of the required amount of the I/O resource of the I/O nodes, and to allocate the secured part of the I/O resource to the job.