RAID and Subsystem Architectures Subsystem Architectures
Cache Protection
• Cache is volatile memory – lost during power-outage • Data in cache is not immediately written to disk
• To protect data in cache, the cache memory has internal battery protection – preserves unwritten data for days until disks are re- powered and data can finally be written to disk.
CACHE CACHE
• High end subsystems also use mirrored cache for high-availability
• Use of Error Checking (ECC) codes can detect & correct memory errors
Remember that data sitting in the cache memory may not have yet been written to the physical disk. Thus, if we get a power failure there is a risk of loss of the changes to the data that have not yet been written to disk.
High end storage arrays use a number of techniques to protect data held in cache;
Battery Protection of Cache memory Mirroring of cached data into two different physical sets of cache memory
Error checking on cache memory, can detect & correct memory errors