Currently if you loose a non-redundant log device in your zpool it becomes impossible to import, there is no way to exclude the log device and import the rest of the pool. There are however light on the horizon, PSARC 2010/292 proposes a solution to this problem with a new switch for the zpool import command.
From the PSARC:
"This fast-track introduce a new command line flag to the
'zpool import' sub-command. This new option, '-m', allows
pools to import even when a log device is missing. The contents
of that log device are obviously discarded and the pool will
operate as if the log device were offlined."
PSARC 2010/292 zpool import despite missing log
This has now been implemented in the source, the changeset is here.
Reflecting on The Soul of a New Machine
1 week ago