Re: [Qemu-devel] [PATCH V4 4/4] Change default to qcow2 for sync mode none.

[Eric Blake]

On 07/17/2013 02:04 PM, Ian Main wrote:
> qcow2 supports backing files so it makes sense to default to qcow2
> for MIRROR_SYNC_MODE_NONE so that we can use the source as a backing
> drive and export it via nbd.  Defaulting FULL and TOP to SYNC_MODE_NONE
> breaks tests but that could be fixed if we wanted it.
> 
> Signed-off-by: Ian Main <address@hidden>
> ---
>  blockdev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 000dea6..805b0e5 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1462,7 +1462,7 @@ void qmp_drive_backup(const char *device, const char 
> *target,
>      }
>  
>      if (!has_format) {
> -        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : 
> bs->drv->format_name;
> +        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : "qcow2";

Is this the right thing to do?  Or should we do:

if (!has_format) {
    if (mode == NEW_IMAGE_MODE_EXISTING) {
        format = NULL;
    } else {
        format = bs->drv->format_name ?: "qcow2";
    }
}

That is, I think we should default to doing a backup in the format given
by the original (what if the original is qed, which also supports
backing files), and only use qcow2 when there is no guidance whatsoever.

But in practice, I don't care - format probing is a security hole, so
libvirt should always be passing a format, at which point the entire
!has_format condition should be skipped when called by libvirt.

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

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