Techniques for utilizing flash storage as an extension of hard disk (HDD) storage are provided. In one embodiment, a computer system stores a subset of blocks of a logical file in a first physical file, associated with a first data structure that represents a filesystem object, on flash storage and a subset of blocks, associated with a second data structure that represents a filesystem object comprising tiering configuration information that includes an identifier of the first physical file, in a second physical file on HDD storage. The computer system processes an I/O request directed to the logical file by directing it to either the physical file on the flash storage or the HDD storage by verifying that the tiering configuration information exists in the data structure and determining whether the one or more blocks are part of the first subset of blocks or the second subset of blocks.