Logical Drive Cache Settings to Protect Cache Memory
Write caching can increase I/O performance, but it requires a battery back-up. Use write cache mirroring to protect data during a controller or cache memory failure. When you enable write cache mirroring, cached data is mirrored across two redundant controllers with the same cache size. (Data left in cache is protected by an onboard cache battery.) The data written to the cache memory of one controller is also written to the cache memory of the other controller. Therefore, if one controller fails, the other can complete all outstanding write operations. Enable the Write Cache Mirroring parameter for each logical drive using the Logical Drive >> Properties option in the Subsystem Management Window.
Sometimes write caching is disabled when batteries are low or discharged. If you enable a parameter called Write Caching Without Batteries on a logical drive, write caching continues even when batteries in the controller enclosure are discharged. If you do not have a Uninterruptible Power Supply (UPS) for power protection, do not enable this parameter because data in the cache will be lost during a power outage when the controller enclosure does not have working batteries.
Storage Subsystem Cache Settings to Protect Cache Memory
To prevent data loss or corruption, the controller periodically writes cache data to disk (flushes the cache). When the amount of unwritten data in cache reaches a certain level, called a start percentage, the software signals the controller to write the data to disk. The controller writes to disk until the amount of data in cache drops to a stop percentage level. For example, you can specify that the controller start flushing the cache when the cache reaches 80% full and stop flushing the cache when the cache reaches 16% full.
For maximum data protection, you can choose low start and stop percentages. However, in both cases this increases the chance that data needed for a host read will not be in the cache, which decreases performance. Choosing low start and stop percentages also increases the number of disk writes necessary to maintain the cache level, increasing system overhead and further decreasing performance.
If a power outage occurs, data in the cache that has not been written to the drive media will be lost, even if it is mirrored to the cache memory of both controllers. For protection against this occurrence, batteries in the controller enclosure protect against power outages. Change the controller enclosure batteries at the recommended time intervals. The storage management software features a battery age clock you can set when you replace a battery. This clock will keep track of the age (in days) of the battery so you know when it is time to replace the battery. You will receive critical alert notification when the battery is nearing expiration and when it has reached expiration.
Specifying Logical Drive Properties