The two power supplies of the S5000 are designed in 1+1 redundant mode. Either of the power supplies can provide sufficient power to the S5000 under standard voltage and current. In the conditions where only one power supply is used to power the S5000, the cache write policy is automatically changed from write back to write through for directly writing cache data to disks when a power failure occurs. This mechanism improves data storage security. Given the write through policy directly writes data to disks without using the cache, the storage performance can be degraded. When both power supplies are used, the cache write policy is changed from write through to write back.