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Re: [libvirt] [PATCH] qemu: snapshot: better error for active external readonly disk



On Mon, Oct 29, 2018 at 13:35:36 +0300, Nikolay Shirokovskiy wrote:
> External snapshot of readonly disk of active domain is impossible but error
> message [1] is cryptic (it's source is qemu). Let's make error message more
> user friendly.
> 
> The problem is libvirtd precreates snapshot image with no write permission which
> is not expected by qemu.
> 
> [1] current error message
> error: internal error: unable to execute QEMU command 'transaction':
>                     Could not create file: Permission denied
> 
> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy virtuozzo com>
> ---
> 
> By the way if domain is not active then snapshot is possible. However top image
> will have write permissions after snapshot. We can make snapshot work for
> active domain I guess if we precreate writable snapshot image, but then we end
> up running readonly disk on writable image. Also making snapshot of readonly
> disk is not that practical so let's just fix error message for now.

We should not allow doing this at all then. Not even when inactive.

>  src/qemu/qemu_driver.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index a52e249..e75931e 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -14683,6 +14683,14 @@ qemuDomainSnapshotPrepare(virDomainObjPtr vm,
>                                                        active, reuse) < 0)
>                  goto cleanup;
>  
> +            if (dom_disk->src->readonly && active) {
> +                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
> +                               _("external snapshot for readonly disk %s "
> +                                 "of active domain is not supported"),
> +                               disk->name);
> +                goto cleanup;
> +            }

Please put this into qemuDomainSnapshotPrepareDiskExternal.
qemuDomainSnapshotPrepare is already too long and we don't need to make
it even longer.

Also as noted I don't think we should allow this even when inactive.

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