From owner-freebsd-fs@FreeBSD.ORG  Thu Nov 26 10:01:04 2009
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Date: Thu, 26 Nov 2009 04:01:02 -0600
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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Subject: kernel update svn 195757->199260, zpool.cache prevents boot
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I upgraded a ZFS system from svn 195757 to 199260 and it would not
boot.  Booting from UFS reported a corrupt pool with many faulted
devices and vdevs.

It turns out that the pool was fine and that zpool.cache was the
problem: it was depending on the drive names being immutable but they
changed between svn 195757 and 199260 for me (and will for many or most
people 7.2->8.0 using the new AHCI driver)

Why not have GEOM record the ZFS GUID of each leaf dev when GEOM
"tastes" the drive (similar to the way GEOM does that for UFS today),
and then use the GUID field from zpool.cache to find the disk instead of
the PATH field?  I don't see that ever failing.