MAGNETIC DISK SUB SYSTEM

PROBLEM TO BE SOLVED: To mitigate the disk neck of a disk array and to improve the system performance by performing a control so as to make data on a second logical volume stay in a cache memory longer than the data on a first logical volume. SOLUTION: In the disk array 2b, the total of the five logical volumes that are one normal capacity logical volume 40d and four reduced capacity logical volumes 40e-40h are arranged. A means capable of specifying upper limit time for monitoring the stay of priority holding data in reduced capacity volume cache memory stay monitoring time provided in a constitution information management table 9 from the outside through a service processor 11 and a host CPU is provided. Also, a priority holding data monitoring control operated during the free time of respective adapters in respective channel adapters(CHA) or disk adapters(DKA) is provided. Then, the control is performed so as to make the data on the second logical volume stay in the cache memory longer than the data on the first logical volume.