Zpool recovery was just integrated into the ONNV gate and will be part of OpenSolaris development build 128 in a about a month. Read more about it in this previous entry Zpool recovery support (PSARC/2009/479).
In short, if you have hardware that does not honor cache flush requests or write ordering (some cheap USB-drives for example) combined with for example loss of power a pool can end up being damaged. This option provided a chance of recovering the pool in a automated way by reverting to a older but sane transaction group.
From the new zpool.1m:
"Recovery mode for a non-importable pool. Attempt to
return the pool to an importable state by discarding the
last few transactions. Not all damaged pools can be
recovered by using this option. If successful, the data
from the discarded transactions is irreversibly lost.
This option is ignored if the pool is importable or
"# zpool clear -F data
Pool data returned to its state as of Tue Sep 08 13:23:35 2009.
Discarded approximately 29 seconds of transactions."
Here is the changeset: http://hg.genunix.org/onnv-gate.hg/rev/8aac17999e4d
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