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Date: Fri, 29 May 2009 05:01:12 -0500
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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To: Andrew Snow <andrew@modulus.org>
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Cc: freebsd-fs@freebsd.org
Subject: Re: ZFS scrub/selfheal not really working
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Andrew Snow wrote:
> It seems like its a good idea to chuck out the whole lot, after first
> double-checking or replacing your controller, cabling, and power
> supply.  ZFS can't help you :-)

No, don't throw bad hardware away!  Save it for testing - it's much
harder to find hardware with intermittent failures that may be useful
for testing than hardware that is dead or doesn't fail in a short amount
of time.

>> So, my question is why doesn't ZFS rewrite those sectors with READ
>> errors during scrub?
>
> Because of the transactional nature of ZFS it writes the fresh data in
> a different part of the disk and then marks the old bad sectors as free.

The old sectors are not freed if they are referenced by snapshots or
clones.  Vacating a block would be a very expensive operation for ZFS in
the general case.