Techniques are described for processing a data operation that writes first data to a data portion currently stored on first physical storage of a first storage tier of a data storage system, including storage tiers each having a different performance ranking relative to other storage tiers. Second physical storage is allocated from one of the storage tiers for storing the first data written to the data portion. This one storage tier is selected independent of the first storage tier. The first data is stored in a cache and destaged from the cache to the second physical storage. A single merged copy of data for the data portion is formed. Based on an activity level of the data portion, a target tier is selected for storing the single merged copy of data for the data portion.