NEC S2900 User Manual

Contents

NEC S2900 User Manual Manual pdf 27 page image

102.2 Write Cache FeatureFor RAID levels 5, 6, or 50, the performance of the disk array unit may decrease during the writing ofsmall amounts of data. This is because the previous data and parity data must be read to recalculate theparity.So, the disk array unit is equipped with cache memory. When write data is stored in the cache memory,the disk array unit terminates the command processing and then writes the data to the disk drive,improving the performance.In general, if the power is shut down before the data saved in the cache memory is written to the diskdrive, the data in the cache memory will be lost. To prevent this, the disk array unit provides a batterybackup to the power supply to retain data in the cache memory.The backup time using the Battery Backup Unit is restricted as follows:The backup time of the battery backup unit depends on the capacity of the installed cache memory andthe service period of the battery. The following table shows the backup time when two battery backupunits are installed and they are both fully charged.The following conditions are imposed to make the write cache preservation feature effective: Two controllers, one or more battery backup units, and power supplies for the battery backup unit(PS0 for BBU0, PS1 for BBU1) are installed and operating normally. The battery backup units are fully charged.If any of the conditions are not satisfied, the write cache preservation feature will not work properly.The batteries installed in the battery backup unit are fully charged for about eight hours.If the battery switch on battery backup unit is not turned on, the battery backup unit does not operate.Battery Back Up unit Service periodCache memory capacity 2 .5 years or under 2.5 to 5 years4GB 96H 48H8GB 72H 36H16GB 48H 24H* The battery backup time can be maintained by replacing the battery at intervals of 2.5 years(with charge).* The life expectancy of a battery is 5 years. However, if the power on/off sequence is requiredeveryday, the batteries are exhausted faster. If so, the batteries should be replaced withcharge at intervals of 2.5 years.