Described are methods, systems, and apparatus, including computer program products for expanding a storage pool-backed file system. A file system expansion trigger is detected. First reserved space in the storage pool is reserved. A LUN size attribute associated with a LUN underlying the file system is increased. A file system size attribute associated with the file system is increased. First data to write to the file system is received. In response to receiving the first data, an expansion result is determined, wherein the expansion result indicates whether to expand the LUN and the file system. If the expansion result indicates to expand the LUN and the file system, a first portion of the first reserved space is provisioned and formatted. The first data is written to the file system.