linux-kernel - Re: [tip:tools/kvm] kvm tools, qcow: Fix copy-on-write image corruption

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Message-ID: <4E27DEA9.20404@redhat.com>
Date:	Thu, 21 Jul 2011 10:09:13 +0200
From:	Kevin Wolf <kwolf@...hat.com>
To:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	levinsasha928@...il.com, penberg@...nel.org, kwolf@...hat.com,
	tglx@...utronix.de
CC:	linux-tip-commits@...r.kernel.org, kvm-devel <kvm@...r.kernel.org>
Subject: Re: [tip:tools/kvm] kvm tools, qcow: Fix copy-on-write image corruption

Am 20.07.2011 21:49, schrieb tip-bot for Pekka Enberg:
> Commit-ID:  4839bcd6b04a1b628f0f00d535ddca2ee872e9fa
> Gitweb:     http://git.kernel.org/tip/4839bcd6b04a1b628f0f00d535ddca2ee872e9fa
> Author:     Pekka Enberg <penberg@...nel.org>
> AuthorDate: Wed, 20 Jul 2011 16:28:31 +0300
> Committer:  Pekka Enberg <penberg@...nel.org>
> CommitDate: Wed, 20 Jul 2011 16:28:31 +0300
> 
> kvm tools, qcow: Fix copy-on-write image corruption
> 
> We don't handle refcount table properly so make sure we only write to clusters
> that have the "copied" flag set.
> 
> Cc: Kevin Wolf <kwolf@...hat.com>
> Cc: Sasha Levin <levinsasha928@...il.com>
> Signed-off-by: Pekka Enberg <penberg@...nel.org>

Hm, I must have missed this patch on the KVM mailing list...

You should also error out before creating a new L2 table as its refcount
would have to be updated, too. On the other hand, the patch is a bit too
restrictive, because it applies the check to qcow1 which doesn't even
have a refcount table.

I'm not sure if everyone is aware of it, so another hint: Now that you
can't grow qcow2 any more, you might find it useful to use qemu-img
create -f qcow2 -o preallocation=metadata, so that all clusters are
already present and you don't get random failures.

Kevin
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