From owner-freebsd-fs@FreeBSD.ORG  Tue Sep 22 14:19:51 2009
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Date: Tue, 22 Sep 2009 08:59:05 -0500
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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To: Kurt Touet <ktouet@gmail.com>
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Cc: freebsd-fs <freebsd-fs@freebsd.org>
Subject: Re: ZFS - Unable to offline drive in raidz1 based pool
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Kurt Touet wrote:
> I wonder then, with the storage array reporting itself as healthy, how
> did it know that one drive had desynced data, and why wouldn't that
> have shown up as an error like DEGRADED?

The uberblock on each ZFS disk contains a txtag, in effect a revision
number.  When a pool is imported and one drive's txtag is older than the
others then that drive needs resilvering.