Programmed Logic Corp. is a company specializing in storage products. Among their offerings are a compression file system, a 64-bit file system, a high-throughput file system utilizing transactions, and a stackable file system. PLC's StackFS [PLC96] is very similar to my wrapper file system described in Appendix sec-appendix-typical-in-core-wrapfs.
StackFS allows for different modules to be plugged in a variety of ways to provide new functionality. Modules offering 64-bit access, mirroring, union, hierarchical storage management (HSM), FTP, Caching, and others are available. Several modules can be loaded in a stack fashion into StackFS. The only organization available is a single stack; that is, each file system performs its task and then passes on the vnode operation to the one it stacked on top of, until the lowest stacked file system access the native file system (UFS or NFS).
There is no support for fan-in or fanout. There is seemingly no support for composition either. Also, StackFS does not have facilities for saving state in an auxiliary file system the way FiST defines Statefs (see Appendix sec-appendix-typical-in-core-statefs). Finally, there is no language available for producing modules that will work within StackFS. Still, PLC's products are the only known commercially available stackable file system implementation.