Re: [Qemu-block] blockdev-commit design

[Kashyap Chamarthy]

On Tue, Sep 26, 2017 at 07:59:42PM +0200, Kevin Wolf wrote:

[...]

> In comparison with the old command, the important changes are:
> 
>   * top/base are node names instead of file names.
> 
>   * You don't need to specify the active layer any more (not the least
>     because there could very well be more than one of them), but top
>     becomes mandatory instead.

As a user of 'block-commit', I'm a bit confused on your above point.
Two questions:

(1) During active block commit, isn't "active layer" == "top"? 

    With the existing QMP `block-commit`, given: 
    
        A <- B <- C <- D (active)
    
    To merge B, C, and D (the active layer) into A, I _have_ to specify
    the active layer, which is "D", as a 'top' parameter:

        {
            "execute": "block-commit",
            "arguments": {
                "device": "node-D",
                "job-id": "job0",
                "top": "d.qcow2",
                "base": "a.qcow2"
            }
        }

    So when merging the top-most layer (D), there's at least one
    scenario where we _are_ specifying the "active layer".  And 'top'
    _is_ mandatory as seen above.  
    
    So I wonder if I'm misinterpreting your wording.

(2) Also, just for my own education, can you mind expanding a bit more
    about the "there can be more than one active layer" scenario?

Thanks.

[...]

-- 
/kashyap