A cache can be hot or cold. A hot cache contains data that is being actively requested (generally because it is in use); a cold cache doesn't contain the data it "should" to serve the purpose for which it is presently being used. Cache warming is the process of going from a cold to a hot cache, and happens as the cache is used. Depending on application, this make take from microseconds to hours; a database cache in a cluster often takes 30 seconds to a couple of minutes to warm after a failover, for instance.