1. A storage subsystem, comprising: an array of non-volatile memory comprising solid state memory devices, said array using programmable voltage reference values to determine states of memory cells within said solid state memory devices; and a controller configured to write data to, and read data from, the array of non-volatile memory in response to commands received from a host system, maintain a table that specifies, for each of a plurality of blocks of the array of non-volatile memory, a respective set of one or more voltage reference values to be used for reading data from the block, and test and update the voltage reference values specified by the table by performing background read operations on the array while monitoring error counts that result from the background read operations performed using specific sets of voltage reference values.