File system cache tiers

Embodiments are directed to a file system engine that provides a file system that includes a plurality of blocks on a file storage tier with portion of the blocks associated with a cache storage tier. A cache engine provides heat extents that include a heat score and a run length value such that the heat extents are represent activities associated with the blocks. Heat extents associated with file system activities are determined based on a position of the blocks that corresponds with the run length values of the heat extents and sectors on the file storage tier. Heat scores associated with the heat extents may be modified based on the activities and distribution models to conform the heat extents with the distribution models. Blocks associated with heat scores that are less than a threshold value provided by the distribution models may be disassociated from the cache storage tier.