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Date: Tue, 8 Nov 2011 09:59:45 -0500
From: Christoph Hellwig <hch@infradead.org>
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Subject: Re: [Qemu-devel] [PATCH] KVM: Add wrapper script around QEMU to
	test kernels
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To: Avi Kivity <avi@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>, "kvm@vger.kernel.org list" <kvm@vger.kernel.org>, qemu-devel Developers <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>, "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>, Christoph Hellwig <hch@infradead.org>, Blue Swirl <blauwirbel@gmail.com>, Am?rico Wang <xiyou.wangcong@gmail.com>, Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@linux-foundation.org>

On Tue, Nov 08, 2011 at 04:57:04PM +0200, Avi Kivity wrote:
> > Running qemu -snapshot on the actual root block device is the only
> > safe way to reuse the host installation, although it gets a bit
> > complicated if people have multiple devices mounted into the namespace.
> 
> How is -snapshot any different?  If the host writes a block after the
> guest has been launched, but before that block was cowed, then the guest
> will see the new block.

Right, thinko - qemu's snapshots are fairly useless due to sitting
ontop of the file to be modified.

> It could work with a btrfs snapshot, but not everyone uses that.

Or LVM snapshot.  Either way, just reusing the root fs without care
is a dumb idea, and I really don't want any tool or script that
encurages such braindead behaviour in the kernel tree.