I don't know a lot about it, but my understanding is that the shim is pretty thin, there to translate some slightly different syscall conventions. Compatibility

I don't know a lot about it, but my understanding is that the shim is pretty thin, there to translate some slightly different syscall conventions. Compatibility shims done well (minimal to no overhead) can be very beneficial, allowing separate projects to share more code directly. On a separate note, illumos is a really cool project. I hope this transition (and apparent reduced activity in their development on ZFS) is not a symptom of the project losing momentum. My first experiences with a computer were on SunOS and early Solaris, and illumos is the the [open source] heir to that throne.