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The level 2 (L2) cache, or back-end cache, refers to local memory on the node on which a particular block of data is stored. L2 cache is globally accessible from any node in the cluster. It is used to reduce the latency of a read operation by not requiring a seek directly from the disk drives. As such, the amount of data prefetched into L2 cache for use by remote nodes is greater than the data in L1 cache.
L2 cache is also known as local cache because it contains data retrieved from disk drives on that node and then is made available for requests from remote nodes. Data in L2 cache is evicted according to a Least Recently Used (LRU) algorithm.
The local node addresses data in L2 cache using an offset into a disk drive that is local to that node. Because the node knows the disk location of the data requested by the remote nodes, this process is a fast way of retrieving data destined for remote nodes. A remote node accesses L2 cache by doing a lookup of the block address for a particular file object. As previously described, MESI invalidation is unnecessary, and the cache is updated automatically during writes and kept coherent by the transaction system and NVRAM.