Write Cache FeatureIn a RAID 5 configuration, the performance of the disk array unit may decrease duringwriting of small amounts of data. It is because the previous and parity data must be readto recalculate the parity.The disk array unit is equipped with cache memory. When write data is stored in thecache memory, the disk array unit terminates the command processing and then writesthe data to disks for improving the performance.Note: The cache memory is subject to battery backup by thebattery backup unit. However, to ensure a higher degree of dataprotection, it is recommended to use a UPS (uninterruptible powersupply).In general, if the power is shut down before the data saved in the cache memory iswritten to disks, the data in the cache memory will be lost. To prevent this, the batterybackup unit retains data in the cache memory.The backup time is restricted as follows: Three days (72 hours) in full charge status of both batteriesThe following conditions must be satisfied to make the write cache feature effective: Two controllers, one or more battery backup units and power supplies for thebattery backup units are installed (factory-set). The battery backup units are fully charged.If any of the conditions is not satisfied, the write cache feature does not workefficiently.The batteries installed in the battery backup unit remain fully charged for about eighthours.1-20 System Overview
