I dug through the mailing list archives and couldn't find a straight answer.
I also checked RCS comment logs on usr.sbin/installboot/installboot.c and
Makefile.  The raidctl(8) is outdated and states:

    "Note that kernels can only be directly read from RAID 1 components on al-
     pha and pmax architectures.  On those architectures, the FS_RAID file
     system is recognized by the bootblocks, and will properly load the kernel
     directly from a RAID 1 component.  For other architectures, or to support
     the root file system on other RAID sets, some other mechanism must be
     used to get a kernel booting.  For example, a small partition containing
     only the secondary boot-blocks and an alternate kernel (or two) could be
     used.  Once a kernel is booting however, and an auto-configuring RAID set"


As best I understand it, on sparc, primary boot blocks are stored in the
first few sectors of the disk.  These are loaded by OpenBOOT which they reads
the UFS (or RAID hopefully) filesystem and sees /boot, the secondary boot
loader.  Please correct me if I'm wrong here >:}

Unfortunately none of this is documented so I'm having some difficulty
getting RAID1 root fs working on sparc.

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