illumos gate

When the system is booting zfs_mountroot() is called to mount the root file system, but if the kernel being booted does not support the pool version of the root file system the system will output a cryptic error message '48' (ENOTSUP) and exit.

zfs_mountroot() eventually calls spa_open() which gives us all of the version incompatibility information we need to display a helpful error message. If the code is re-organized a bit we could do the call to spa_open() in a way that would allow us to retain the version information we need to output a useful error message.