freebsd stable - Aug 2013 - Boot problem if a ZFS log device is missing

Hi all,
I am using an USB memory stick as cache and log devices for a HDD ZFS 
pool named tank0:
$ zpool status -v tank0
   pool: tank0
  state: ONLINE
   scan: scrub repaired 0 in 7h19m with 0 errors on Tue Jul 30 06:11:23 2013
config:

     NAME         STATE     READ WRITE CKSUM
     tank0        ONLINE       0     0     0
       ada0s1d    ONLINE       0     0     0
     logs
       gpt/SLOG   ONLINE       0     0     0
     cache
       gpt/L2ARC  ONLINE       0     0     0

errors: No known data errors

If I remove the stick before booting the laptop (a Compaq Presario) it 
will not boot with the error message:
"Mounting from zfs:tank0 failed with error 6"
end the mountroot> prompt is displayed.
I need to reinsert the stick to completing the boot process before 
restarting the laptop.

It is possible to boots the laptop without stick if I remove the log 
device with the command:
# zpool remove tank0 gpt/SLOG
before switching it off. The pool will works in degraded mode because 
the cache device is missing, but it works.

I am able to boot the PC without a cache device but not without a log 
device. Why ?
The laptop is updated to 9.2-PRERELEASE r254783.

Regards
Maurizio

on 29/08/2013 11:27 Maurizio Vairani said the following:
> 
> I am able to boot the PC without a cache device but not without a log
device. Why ?
The log could potentially contain uncommitted entries.  Without the log device
there is no knowing if it did or did not.  And if it did then the pool is
inconsistent state without the log device and so it can not be imported.

The cache is not persistent and so there is nothing needed from it upon a boot.

-- 
Andriy Gapon