A write-back cache is more complex to implement, since it needs to track which of its locations have been written over, and mark them as "dirty" for later writing to the backing store. The data in these locations are written back to the backing store only when they are evicted from the cache, an effect referred to as a "lazy write". For this reason, a read miss in a write-back cache (which requires a block to be replaced by another) will often require two memory accesses to service: one to write the replaced data from the cache back to the store, and then one to retrieve the needed data.