Operational information in a storage system is collected regarding storage media storage tiers, devices, drives, tracks on drives, and logical storage layers, to determine an estimated amount of time it will take to write data from cache to the intended drive when a new write operation arrives at the storage system. This information is then used to decide which type of cache is most optimal to store the data for the write operation, based on the estimated amount of time it will take to write data out from the cache. By allocating cache slots from a faster cache to write operations that are expected to quickly be written out to memory, and allocating cache slots from the slower cache to write operations that are expected to take more time to be written out to memory, it is possible to increase the availability of the cache slots in the faster cache.