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From: Eric Blake <eblake@redhat.com>
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Date: Tue, 3 Nov 2015 08:14:57 -0700
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Subject: Re: [Qemu-devel] [PATCH v3 1/3] block: Disallow snapshots if the
 overlay doesn't support backing files
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To: Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>

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On 11/03/2015 03:32 AM, Alberto Garcia wrote:
> This addresses scenarios like this one:
>=20
>   { 'execute': 'blockdev-add', 'arguments':
>     { 'options': { 'driver': 'qcow2',
>                    'node-name': 'new0',
>                    'file': { 'driver': 'file',
>                              'filename': 'new.qcow2',
>                              'node-name': 'file0' } } } }
>=20
>   { 'execute': 'blockdev-snapshot', 'arguments':
>     { 'node': 'virtio0',
>       'overlay': 'file0' } }
>=20
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  blockdev.c | 5 +++++
>  1 file changed, 5 insertions(+)
>=20

> +++ b/blockdev.c
> @@ -1667,6 +1667,11 @@ static void external_snapshot_prepare(BlkTransac=
tionState *common,
> =20
>      if (state->new_bs->backing !=3D NULL) {
>          error_setg(errp, "The snapshot already has a backing image");
> +        return;
> +    }
> +
> +    if (!state->new_bs->drv->supports_backing) {
> +        error_setg(errp, "The snapshot does not support backing images=
");

If we do s/snapshot/overlay/ here, the error message will make more
sense (I noticed it in 3/3).

My R-b stands either way, though.

--=20
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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