An in-core file system is the type that has been developed so far in this proposal. State is maintained by the interposing vnodes.
The main attraction of an in-core file system is that its state may be regenerated after an unmount, reboot, or crash. In general, the state of the file system can be recovered by simply remounting it. A secondary advantage to in-core file systems is their simplicity. With just a few small modifications to Wrapfs one can generate many interesting and useful file systems, as exemplified in Appendix sec-appendix-typical-in-core. I expect many file systems generated by FiST to fall into this category.