Controlling write sizes to reduce flash wear

A technique for managing storage in a data storage system sends a write from a second file system to a first file system at block-level granularity of the first file system and provides a set of flags that identify which flash-page-size portion(s) of a block are being changed. If the write is directed to a data block that is not shared, such that no write split is required, the improved technique checks the set of flags and proceeds to overwrite the data block in the first file system with only the flash-page-size portion(s) that have changed. The improved technique thus performs the overwrite in flash-page-size increments, avoiding writes to flash pages that remain unchanged.