Some facts about disk cache

acroman

Storage System Cache Modes
* Read Caching
  – Reads are kept in cache
* Read Ahead Caching
  – Prefetch specified as either # of Blocks, or a multiple of the read request
  – The controller issues another I/O to the drives and caches the data
* Write-Through Caching
  – Status is not returned to the host until the data is stored to non-volatile media
* Write-Back Caching
  – Write command status is returned to the host as soon as the data has been inserted into the cache pool
* Non-Volatile Write-Back Caching (Cache Mirroring)
  – Write-Back Cache extension that mirrors data to the second controller for redundancy
Cache Flush Algorithms
* Age Based
  – User specifies the period of time dirty write data should remain in cache before it is scheduled to be flushed to media
* Demand Based
  – Used after the cache amount crosses the specified threshold boundary
  – Proactively flushes dirty write cache to accommodate higher levels of cache usage
  – Priority is given to full stripes
  – Used until cache amount goes back below threshold
  – Demand Mode Takes Priority
       
Cache Block Size
* 4 KB (8 - 512 byte blocks)  --> Small random I/O
* 16 KB (32 - 512 byte blocks) --> Large sequential I/O


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