8.2->9.prerel: gmirror failed with error 19

[Randy Bush]

8.2 system fully updated as of 2011.12.14
it can reboot quite happily

csup to RELENG_9
make buildworld
make kernel
boot single

    root mount waiting for: usbus4
    uhub4: 8 ports with 8 removable, self powered
    Trying to mount root from ufs:/dev/mirror/boota [rw]...
    mountroot: waiting for device /dev/mirror/boota ...
    Mounting from ufs:/dev/mirror/boota failed with error 19.

neither 9 nor 8 would boot without ending up here

the only way out was via loader

    OK unload
    OK load boot/kernel.old/kernel
    OK load boot/kernel.old/geom_mirror.ko
    OK set kern.geom.part.check_integrity=0
    OK set vfs.root.mountfrom.options=rw
    OK boot -s

this would only work with the 8.2 /boot/kernel.old but not with the
9.fresh /boot/kernel

randy