34 Planning: RAID Controller ModulesCache Functions and FeaturesCache MirroringCache mirroring function copies accepted host-write data from the primarycontroller to the partner controller. This action ensures that host-write data issafely mirrored to the partner controller before successful completion status isreturned to the host. If a controller fails, the surviving controller safely retainsall mirrored data. Cache mirroring is enabled by default.Write-Back CacheIn Write-back Cache, write operations result in a completion signal beingsent to the host operating system as soon as the cache receives the data to bewritten. The target physical disk receives the data at a more appropriate timein order to increase controller performance. In dual-active controllerconfigurations with Write-back Caching enabled, the write data is alwaysmirrored to the cache of the second controller before completion status isissued to the host initiator. Write-Back Cache is enabled by default unlesscache mirroring is disabled.Write-Through CacheIn write-through cache, data is written to the physical disk before completionstatus is returned to the host operating system. Write-through cache isconsidered more robust than write-back cache, since a power failure is lesslikely to cause loss of data. The RAID controller automatically switches towrite-through if cache mirroring is disabled or if the battery is missing or has afault condition.
