This issue is caused because of a problem with the way synchronous I/O is handled when a file system is unmounted. When more than one ZFS BE exists in a ZFS root pool, a recursive snapshot might fail.
cannot create snapshot 'rpool@today': dataset is busy
Workaround: Choose one of the following workarounds:
Workaround 1: Mount and unmount the file systems that are indicated as busy in the error messages.
Workaround 2: Remove any additional ZFS BEs before creating a recursive snapshot of a ZFS root pool.