# Jeb Campbell <[email protected]>
NOTE: This is last resort if you need your data now.  This worked for me, and 
I hope it works for you.  If you have any reservations, please wait for Sun 
to release something official, and don't blame me if your data is gone.

PS -- This worked for me b/c I didn't try and replace the log on a running 
system.  My log got borked on a system crash, but others have had data loss
after trying to zpool replace the log device. 

Please compile (and read the source) if you can, but just in case:
md5sum logfix
fc00c9494769abbc4e309d2efb13d11b  logfix

Currently (as of 6/5/2008) if a log device gets wiped or lost from a pool, 
you are no longer able to import the pool.

The technical reason is that the log device info is not stored anywhere else, 
so if you try to import the pool, the checksum of vdevs won't match.

Perhaps ZFS shouldn't use log and cache devices in computing the pool's 
checksum...

Here is what you will need to recover:

** /etc/zfs/zpool.cache from your install **
To get this, I had to boot from a livecd, then:
# zpool import -f rpool
# mount -F zfs rpool/ROOT/opensolaris /mnt
# cp /mnt/etc/zfs/zpool.cache /mnt/etc/zfs/zpool.cache.log
Then you can copy that somewhere else, but save that copy...

** guid of log device **
The *only* place that guid exists is zpool.cache.log (assuming the log
device is wiped.  So stash that file 5 places if you need to...
Back to extracting the guid, this worked for me:
# cp /mnt/etc/zfs/zpool.cache.log /etc/zfs
# cd /etc/zfs
# mv zpool.cache zpool.cache.running; cp zpool.cache.log zpool.cache; \
	zdb -C > cache_dump; cp zpool.cache.running zpool.cache
We just slipped the saved file in, dump it, then restore the old one.
(I think zdb can load an alt cache file, but I couldn't do it)
Now examine the cache_dump file and find your pool, then log device.
We are looking for the "guid" of the log device.  Once you have it,
again save it 10 places (and not on the livecd).

** format your new log device **
You still need a log device to bring this back up.  Anything will do
if you are going to migrate your data off this setup.  I chose to keep
my log device, now that I have a way to restore it.
# cd /tmp
# dd if=/dev/zero of=junk bs=1024k count=64
# zpool create junkpool /tmp/junk log your_new_log_device
# zpool export junkpool

** Find your old disks **
You need one of your pool's devices to read the vdev_label.  It will
generally look something like this /dev/rdsk/cXtXdXs0 (or cXdXs0).
Check out the label with:
# zdb -l /dev/rdsk/cXtXdXs0
(You can also find your old disks with zpool import on a livecd)

** Fix it up! **
For disk based log devices:
# ./logfix /dev/rdsk/cXtXdXs0 /dev/rdsk/${your_new_log_device}s0 guid
For file based log devices (this will be slow -- get the data off...):
# ./logfix /dev/rdsk/cXtXdXs0 /path/your_new_log_file guid

** Import your pool **
Please before we go any further, COPY YOUR GUID SOMEWHERE SAFE!!!
# zpool import -f pool
This might take a while as ZFS does it's thing and checks everything out.

I hope everything went ok! -- Jeb