"BTX halted" when booting 9.0-BETA3 (Root On ZFS)

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Hello,

For a couple of days now, I can't boot FreeBSD 9-BETA3 on my laptop.
It stops at this stage (copied by hand from a screenshot):

  FreeBSD/x86 ZFS enabled bootstrap loader, Revision 1.1
  (root_at_farrell.cse.buffalo.edu, Sat Sep 24 20:25:50 UTC 2011)
  -
  int=00000000  err=00000000  efl=00010246  eip=0002f4ab
  eax=00000001  ebx=00000000  ecx=00000000  edx=00000000
  esi=00000000  edi=00000000  ebp=00094880  esp=00094808
  cs=002b  ds=0033  es=0033    fs=0033  gs=0033  ss=0033
  cs:eip=f7 f1 85 db 89 c1 89 45-94 74 08 8b 55 18 89 32
         89 7a 04 89 4d 98 8b 45-94 8b 55 98 83 c4 6c 5b
  ss:eip=00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00
         00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00
  BTX halted

It was installed using the RootOnZFS guide almost two years ago (MBR,
multiboot with Windows 7 and Ubuntu, ZFS in a single slice[1]). I'm
usually tracking CURRENT up until the ports breakage with 10-CURRENT.
Now I'm tracking 9.0.

I built world from SVN revision 226141. But now, kernel, zfsboot and
zfsloader are those from 9.0-BETA3's DVD1. The zpool is version 28 and
the zfs filesystems are version 5.

It all started when I copied a directory containing around 3.5GB of
JPEG images, while building world (I can't remember if the build was
finished, maybe it was waiting for install). This was quite slow (but
I can't give you numbers). When I then ran make installkernel, I found
it to be really slow two (maybe 1-2 seconds per module). I continued
with installworld in single user, then rebooted.

I can't remember if the problems appeared right away but the first one
was an "ZFS: invalid zap_type=134218628" at boot, exactly as described
on freebsd-fs_at_ [2].

At this point, I used VirtualBox (on Windows) to boot from 9.0-BETA3's
DVD1 and restored kernel.old (I think). Now I have this "BTX halted"
exception.

What I tried so far:
    o  reinstall zfsboot from 9.0-BETA3
    o  restore zfsloader.old
    o  reinstall zfsloader from 9.0-BETA3
    o  zfs scrub (no error)
    o  regen zpool.cache
    o  SMART Extended Text (no error)
    o  memtest (no error)
    o  zdb -cv <pool> but it's unable to complete, not enough memory

The only sign of hardware failure is "Reallocated Sector Count: 98"
from smartctl. But for now, I can't be sure the HDD is the root of the
problems.

Does someone have any suggestions about this sofware and/or hardware
issue? I'm running out of ideas on how to isolate it.

[1] http://wiki.freebsd.org/RootOnZFS/ZFSBootPartition
[2] http://lists.freebsd.org/pipermail/freebsd-fs/2011-August/012248.html

- -- 
Jean-Sébastien Pédron
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