Re: [Qemu-devel] [PATCH v2 02/10] block: set snapshot option for block devices in blkreplay module

[Paolo Bonzini]

On 15/09/2016 11:00, Pavel Dovgalyuk wrote:
> diff --git a/docs/replay.txt b/docs/replay.txt
> index 347b2ff..5be8f25 100644
> --- a/docs/replay.txt
> +++ b/docs/replay.txt
> @@ -196,6 +196,14 @@ is recorded to the log. In replay phase the queue is 
> matched with
>  events read from the log. Therefore block devices requests are processed
>  deterministically.
>  
> +blkdriver also supports overlay option, which allows creating persistent
> +overlay file for saving and reloading VM snapshots in record/replay modes.
> +Replay mechanism automatically creates one snapshot named 'replay_init' to
> +allow rewinding execution while replaying.
> +Overlay file may be specified as follows:
> + -drive driver=blkreplay,if=none,image=img-direct,
> +        overlay=overlay.qcow2,id=img-blkreplay

So in this case the image is actually overlay.qcow2, and it is created 
with img-direct as the backing file?  Since you have to create 
overlay.qcow2 outside QEMU anyway, overlay.qcow2 might as well be the 
"image".  That is, you could choose between:

   -drive driver=blkreplay,if=none,image=overlay.qcow2,id=img-blkreplay \
   -rr snapshot=replay_init,...

   -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay

The temporary snapshot would be created if there's no "-rr snapshot" option
on the command line.

Does this make sense?

Paolo

> 
> +static QemuOptsList blkreplay_runtime_opts = {
> +    .name = "quorum",

Pasto. ;)

> +    .head = QTAILQ_HEAD_INITIALIZER(blkreplay_runtime_opts.head),
> +    .desc = {
> +        {
> +            .name = "overlay",
> +            .type = QEMU_OPT_STRING,
> +            .help = "Optional overlay file for snapshots",
> +        },
> +        { /* end of list */ }
> +    },
> +};