Internal Cache Layer

In this page, we will explain how we design and implement I/O buffer model (data cache), named Internal Cache Layer - ICL.

ICL provides abstract class, so you can implement your own data buffer model by inherit the class. By default, we provide GenericCache, which implements set-associative cache.

../_images/icl.jpg

Components of Internal Cache Layer

Abstract Class

AbstractCache class is defined in icl/abstract_cache.hh. It defines five virtual functions (+ three virtual functions derived from StatObject).

ICL get I/O requests from each virtual function and buffer I/O with own algorithm. Pass I/O to FTL when data eviction needed or flush request arrived.

Data buffer in SSD is very important for hiding slow NAND I/O performance. Small changes to the buffer algorithm introduces big performance changes.

Set-Associative Cache

We provides set-associative cache, which defined as SimpleSSD::ICL::GenericCache in icl/generic_cache.hh. Following parameters are customizable. For more detailed description, check sample configuration file.

  • CacheSize: Buffer capacity.
  • CacheWaySize: Set associativity of cache. Set size will automatically calculated.
  • EnableReadCache: Enable read data caching.
  • EnableReadPrefetch: Enable read-ahead and prefetch.
  • ReadPrefetchMode: Specify how many data should be read-ahead/prefetch.
  • ReadPrefetchCount: Threshold for read-ahead/prefetch (# of sequential I/O).
  • ReadPrefetchRatio: Threshold for read-ahead/prefetch (data size of sequential I/O).
  • EnableWriteCache: enable write data caching.
  • EvictPolicy: Specify which algorithm to use to select victim cache line.
  • EvictMode: Specify how many data should be evicted when cache is full.
  • CacheLatency: Set cache metadata access latency.