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Description of problem: The --rootfs and --rootdev parameters are needed in order to create an initrd capable of nfsroot/netboot. These flags aren't present when looking at command help (--help) or viewing the man page. How reproducible: mkinitrd --help / man mkinitrd Expected results: Information about the usage and implications of --rootfs/--rootdev parameters. Additional information: BZ 244727 and BZ 243775 contain additional nfsroot information.
mkinitrd is a dead end at this point. RHEL-6 and beyond have moved to dracut. While I agree that the documentation for mkinitrd is not stellar, this is not really appropriate for the 5.10 update release.