From owner-freebsd-fs@FreeBSD.ORG  Sat Sep 22 16:28:09 2012
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Date: Sat, 22 Sep 2012 19:28:05 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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Subject: zfs: allow to mount root from a pool not in zpool.cache
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Currently FreeBSD ZFS kernel code doesn't allow to mount root filesystem on a
pool that is not listed in zpool.cache as only pools from the cache are known to
ZFS at that time.

This patch is an attempt to improve the behavior:
http://people.freebsd.org/~avg/spa_import_rootpool.diff

This could be useful when importing pools that were exported from other systems.
There is a tunable vfs.zfs.rootpool.prefer_cached_config which is set to 1 by
default.  1 means just use a cached pool config if it's found in the cache, 0
means to re-probe disks and read supposedly latest/actual config in any case.

-- 
Andriy Gapon