1. A storage system comprising: a random access memory facility; a cache formed from non-volatile solid-state storage and designated to cache data evicted from the random access memory facility, the non-volatile solid-state storage including a plurality of erase blocks; a processor; and a memory storing instructions for execution by the processor to cause the storage server to execute operations including monitoring usage of a first data in the storage system; based on results of said monitoring, classifying the first data into a plurality of data classes according to usage characteristics of the first data, each of the data classes being associated with a different one of a plurality of cache replacement policies applied to the cache, the plurality of data classes including a first class for data which are expected to remain valid in the cache for at least a complete cycle of a circular FIFO cache replacement log and a second class for data which are expected to remain valid in the cache for less than a complete cycle of the cache replacement log, one of the cache replacement policies being to replace a second data from the first class upon expiration of a complete cycle of the cache replacement log in an event that the second data are determined not to be recently used; and selecting storage locations in the cache for the first data, based on results of said classifying, including confining at least some of the first data in each of the data classes to a specified set of one or more of said erase blocks, wherein data of one erase block of the plurality of erase blocks is subject to a cache replacement policy that is the same for all data of the one erase block.