Learn About Cache Memory Protection

Virtual Disk Cache Settings to Protect Cache Memory

 can increase I/O performance, but it requires a battery back-up. Use  to protect data during a  or  failure. When you enable write cache mirroring, cached data is mirrored across two redundant RAID controller modules 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 RAID controller module is also written to the cache memory of the other RAID controller module. Therefore, if one RAID controller module fails, the other can complete all outstanding write operations. Enable the Write cache mirroring parameter for each  by selecting Virtual Disk >> Change >> Cache Settings in the .

Sometimes write caching is disabled when batteries are low or discharged. If you enable the parameter Write caching without batteries on a virtual disk, write caching continues even when batteries in the RAID controller module enclosure are discharged. If you do not have a Uninterruptible Power Supply (UPS) for power protection, or if you do not have RAID controller module batteries, and Write caching without batteries is enabled, do not enable this parameter because data in the cache will be lost during a power outage when the RAID controller module enclosure does not have working batteries.

Storage Array Cache Settings to Protect Cache Memory

To prevent data loss or corruption, the RAID controller module 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 RAID controller module to write the data to disk. The RAID controller module writes to disk until the amount of data in cache drops to a stop percentage level. For example, you can specify that the RAID controller module 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  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 physical disk media will be lost, even if it is mirrored to the cache memory of both RAID controller modules. For protection against this occurrence, batteries in the RAID controller module enclosure protect against power outages. Change the RAID controller module 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.

Related Topics

Changing Storage Array Cache Settings

Changing Virtual Disk Properties

Learn About Cache Settings and Performance