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Date: Mon, 16 Dec 2013 17:04:28 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20131216160428.GC16756@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] sheepdog: fix dynamic grow for running
 qcow2 format
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To: Liu Yuan <namei.unix@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, sheepdog@lists.wpkg.org, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

On Sat, Dec 14, 2013 at 01:29:28AM +0800, Liu Yuan wrote:
> When running qcow2 over sheepdog, we might meet following problem
> 
>   qemu-system-x86_64: shrinking is not supported
> 
> And cause IO errors to Guest. This is because we abuse bs->total_sectors, which
> is manipulated by generic block layer and race with sheepdog code.
> 
> We should directly check if offset > vdi_size to dynamically enlarge the volume
> instead of 'offset > bs->total_sectors', which will cause problem when following
> case happens:
> 
>    vdi_size > offset > bs->total_sectors
> 
>    # then trigger sd_truncate() to shrink the volume wrongly.
> 
> Cc: qemu-devel@nongnu.org
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Reported-by: Hadrien KOHL <hadrien.kohl@gmail.com>
> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> ---
>  block/sheepdog.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)

block.c already updates bs->total_sectors if bs->growable so this patch
is safe.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan