linux-kernel - Re: [PATCH] KVM: Add wrapper script around QEMU to test kernels

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

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.
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