ZFS: i/o error all block copies unavailable

[Ruslan Yakovlev]

Hi all
After power down on FreeBSD 8.2-STABLE #6 (now updated to #7, but 
problem standing) I can't boot from ZFS v28.
gptzfsboot wrote
   boot: ZFS: i/o error all block copies unavailable
instead
   boot: qroot:/boot/kernel/kernel
I download FreeBSD 9.0-BETA1 image and boot from it. I can mount my ZFS 
storage. I copy /boot from ZFS storage to flash and now kernel booted 
from flash fine, after that ZFS storage mounted as / and all work. zpool 
scrub don't detect any problems. zpool status wrote "No known data errors".
But it too slowly and I want normally boot from ZFS storage without 
loading kernel from flash. How can I fix "ZFS: i/o error all block 
copies unavailable" ?

Now I have
FreeBSD QUAZIS.SNNLAN.local 8.2-STABLE FreeBSD 8.2-STABLE #7: Fri Aug 12 
23:27:33 EEST 2011 root at QUAZIS.SNNLAN.local:/usr/obj/usr/src/sys/main8 i386

=> 34 976770988 ad4 GPT (465G)

34 256 1 freebsd-boot (128k)

290 16777216 2 freebsd-swap (8.0G)

16777506 959993516 3 freebsd-zfs (457G)