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Employing an efficient cache eviction or replacement policy is critical for cache performance. This efficiency is evident in OneFS, where each level of the cache hierarchy uses a different methodology for eviction, to suit the attributes of that cache type. For L1 cache in storage nodes, cache aging is based on a drop-behind algorithm. The L2 cache uses a Least Recently Used (LRU) algorithm because it is relatively simple to implement, uses little overhead, and performs well in general. By contrast, the L3 cache employs a first-in, first-out (FIFO) eviction policy because it is writing to what is effectively a specialized linear file system on SSD.
For OneFS, a drawback of LRU is that it is not scan-resistant. For example, a OneFS Job Engine job or backup process that scans a large amount of data can cause the L2 cache to be flushed. As we will see, this problem can be mitigated to a large degree by the L3 cache. Other eviction policies can promote frequently accessed entries such that they are not evicted by scanning entries, which are accessed only once.