An in-line (or foreground) approach to obtaining contiguous ranges of free space in a file system of a data storage system that can select windows having blocks suitable for relocation at a time when one or more blocks within the respective windows are freed or de-allocated. By providing the in-line or foreground approach to obtaining contiguous ranges of free space in a file system, a more efficient determination of windows having blocks suitable for relocation can be achieved, thereby conserving processing resources of the data storage system.